diff --git a/docs/how-to/test-fixtures.md b/docs/how-to/test-fixtures.md
index 1f693d3..adec5f8 100644
--- a/docs/how-to/test-fixtures.md
+++ b/docs/how-to/test-fixtures.md
@@ -27,6 +27,24 @@ Connection tasks belong to their fixture's accept loop. Stopping the fixture
 cancels and joins them; dropping it cancels the owner. Git fixture subprocesses
 are cancelled with their request, and all three pipes are driven concurrently.
 
+## Shared repository identity
+
+`setup_announcement_on_relay` returns an announcement and a `RepositoryFixture`.
+Prepare it once for each logical repository, then call `fixture.install(&relay)`
+for every other relay. It reuses the signed announcement, signed state event,
+and Git commit. Repeating setup independently can create competing replaceable
+events at the same owner/identifier and different Git histories.
+
+List all participating domains when preparing the fixture. If historic tests
+must seed a source before starting the syncing relay, reserve the target's
+listener first and transfer it into the target after seeding. Do not use sleeps
+or fabricated future timestamps to order copies of the same fixture. Tests of
+actual repository revisions should construct those revisions explicitly.
+
+The sync regression `reinstalling_shared_repository_preserves_event_ids_and_git_refs`
+checks exact announcement/state IDs and remote Git refs across installation and
+reinstallation on two relays.
+
 ## Time and streaming assertions
 
 Age private cache timestamps explicitly and bracket wall-clock timestamps
diff --git a/tests/common/sync_helpers.rs b/tests/common/sync_helpers.rs
index f448f2c..55e4fb0 100644
--- a/tests/common/sync_helpers.rs
+++ b/tests/common/sync_helpers.rs
@@ -1351,100 +1351,121 @@ pub async fn push_unique_git_data_to_relay(
     git_temp_dir
 }
 
-/// Set up a repository announcement on a relay with git data so it passes purgatory.
+/// One repository's immutable Git data and signed events for all test relays.
 ///
-/// With the announcement purgatory feature, announcements (kind 30617) require git
-/// data before they are promoted to the relay's main DB. This helper:
-///
-/// 1. Creates a local git repo with a commit
-/// 2. Builds an announcement and state event (kind 30618) pointing to the relay
-/// 3. Sends both to the relay (they go to purgatory)
-/// 4. Git pushes to the relay → releases both from purgatory immediately
-/// 5. Returns the announcement event and temp dir (keep alive for test duration)
-///
-/// # Arguments
-/// * `relay` - The relay to set up the announcement on
-/// * `keys` - Keys to sign the announcement with (repo owner)
-/// * `domains` - All domains that should be listed in the announcement (including relay.domain())
-/// * `identifier` - Repository identifier (d-tag)
-///
-/// # Returns
-/// `(Event, tempfile::TempDir)` - The announcement event and temp dir.
-/// The temp dir MUST be kept alive for the duration of the test.
+/// Preparing separate copies can mint competing replaceable events for the same
+/// owner/identifier and different Git commits. Prepare once, list every endpoint
+/// (reserve its listener first if necessary), and install this fixture on each
+/// relay. Keep it alive until the test finishes.
+pub struct RepositoryFixture {
+    git_dir: tempfile::TempDir,
+    announcement: Event,
+    state_event: Event,
+    keys: Keys,
+    identifier: String,
+}
+
+impl RepositoryFixture {
+    pub fn new(keys: &Keys, domains: &[&str], identifier: &str) -> Self {
+        use super::purgatory_helpers::{
+            create_state_event, create_test_repo_with_commit, CommitVariant,
+        };
+
+        let npub = keys
+            .public_key()
+            .to_bech32()
+            .expect("Failed to convert public key to npub");
+
+        // Create local git repo with a commit
+        let git_temp_dir = tempfile::tempdir().expect("Failed to create temp dir for git repo");
+        let commit_hash =
+            create_test_repo_with_commit(git_temp_dir.path(), CommitVariant::StateTest)
+                .expect("Failed to create test git repo");
+
+        // Build clone URLs and relay URLs from domains
+        let clone_urls: Vec<String> = domains
+            .iter()
+            .map(|d| format!("http://{}/{}/{}.git", d, npub, identifier))
+            .collect();
+        let relay_urls: Vec<String> = domains.iter().map(|d| format!("ws://{}", d)).collect();
+
+        // Build announcement event (lists ALL domains for relay discovery)
+        let announcement = EventBuilder::new(Kind::GitRepoAnnouncement, "Repository state")
+            .tags(vec![
+                Tag::identifier(identifier),
+                Tag::custom("clone", clone_urls.clone()),
+                Tag::custom("relays", relay_urls.clone()),
+            ])
+            .finalize(keys)
+            .expect("Failed to sign repo announcement");
+
+        // Build state event with all domains' clone URLs
+        let state_event = create_state_event(
+            keys,
+            identifier,
+            &[("main", &commit_hash)],
+            &[],
+            &clone_urls.iter().map(|s| s.as_str()).collect::<Vec<_>>(),
+            &relay_urls.iter().map(|s| s.as_str()).collect::<Vec<_>>(),
+        )
+        .expect("Failed to create state event");
+
+        Self {
+            git_dir: git_temp_dir,
+            announcement,
+            state_event,
+            keys: keys.clone(),
+            identifier: identifier.to_string(),
+        }
+    }
+
+    pub fn path(&self) -> &std::path::Path {
+        self.git_dir.path()
+    }
+
+    /// Publish the same events and push the same commit to another relay.
+    pub async fn install(&self, relay: &TestRelay) {
+        use super::purgatory_helpers::push_to_relay;
+
+        send_to_relay(relay, &self.announcement)
+            .await
+            .expect("Failed to send announcement");
+        send_to_relay(relay, &self.state_event)
+            .await
+            .expect("Failed to send state event");
+        let npub = self.keys.public_key().to_bech32().expect("owner npub");
+        push_to_relay(self.path(), &relay.domain(), &npub, &self.identifier)
+            .expect("Failed to push git data to relay");
+
+        for (label, event) in [
+            ("state", &self.state_event),
+            ("announcement", &self.announcement),
+        ] {
+            let visible = wait_for_event_on_relay(
+                relay.url(),
+                Filter::new().id(event.id),
+                Duration::from_secs(10),
+            )
+            .await;
+            if !visible {
+                report_setup_visibility_failure(relay, &self.keys, &self.identifier, event).await;
+            }
+            assert!(visible, "pushed {label} must leave purgatory");
+        }
+    }
+}
+
+/// Prepare and install a repository on its first relay. For another relay,
+/// call `install` on the returned fixture instead of preparing a new copy.
 pub async fn setup_announcement_on_relay(
     relay: &TestRelay,
     keys: &Keys,
     domains: &[&str],
     identifier: &str,
-) -> (Event, tempfile::TempDir) {
-    use super::purgatory_helpers::{
-        create_state_event, create_test_repo_with_commit, push_to_relay, CommitVariant,
-    };
-
-    let npub = keys
-        .public_key()
-        .to_bech32()
-        .expect("Failed to convert public key to npub");
-
-    // Create local git repo with a commit
-    let git_temp_dir = tempfile::tempdir().expect("Failed to create temp dir for git repo");
-    let commit_hash = create_test_repo_with_commit(git_temp_dir.path(), CommitVariant::StateTest)
-        .expect("Failed to create test git repo");
-
-    // Build clone URLs and relay URLs from domains
-    let clone_urls: Vec<String> = domains
-        .iter()
-        .map(|d| format!("http://{}/{}/{}.git", d, npub, identifier))
-        .collect();
-    let relay_urls: Vec<String> = domains.iter().map(|d| format!("ws://{}", d)).collect();
-
-    // Build announcement event (lists ALL domains for relay discovery)
-    let announcement = EventBuilder::new(Kind::GitRepoAnnouncement, "Repository state")
-        .tags(vec![
-            Tag::identifier(identifier),
-            Tag::custom("clone", clone_urls.clone()),
-            Tag::custom("relays", relay_urls.clone()),
-        ])
-        .finalize(keys)
-        .expect("Failed to sign repo announcement");
-
-    // Build state event with all domains' clone URLs
-    let state_event = create_state_event(
-        keys,
-        identifier,
-        &[("main", &commit_hash)],
-        &[],
-        &clone_urls.iter().map(|s| s.as_str()).collect::<Vec<_>>(),
-        &relay_urls.iter().map(|s| s.as_str()).collect::<Vec<_>>(),
-    )
-    .expect("Failed to create state event");
-
-    // Send announcement and state event to relay (both go to purgatory)
-    send_to_relay(relay, &announcement)
-        .await
-        .expect("Failed to send announcement");
-    send_to_relay(relay, &state_event)
-        .await
-        .expect("Failed to send state event");
-
-    // Git push to relay → releases both from purgatory
-    push_to_relay(git_temp_dir.path(), &relay.domain(), &npub, identifier)
-        .expect("Failed to push git data to relay");
-
-    for (label, event) in [("state", &state_event), ("announcement", &announcement)] {
-        let visible = wait_for_event_on_relay(
-            relay.url(),
-            Filter::new().id(event.id),
-            Duration::from_secs(10),
-        )
-        .await;
-        if !visible {
-            report_setup_visibility_failure(relay, keys, identifier, event).await;
-        }
-        assert!(visible, "pushed {label} must leave purgatory");
-    }
-
-    (announcement, git_temp_dir)
+) -> (Event, RepositoryFixture) {
+    let fixture = RepositoryFixture::new(keys, domains, identifier);
+    fixture.install(relay).await;
+    (fixture.announcement.clone(), fixture)
 }
 
 /// Diagnose a failed exact-ID check without extending its success deadline.
diff --git a/tests/sync.rs b/tests/sync.rs
index 9ce8a73..bd9b66d 100644
--- a/tests/sync.rs
+++ b/tests/sync.rs
@@ -46,6 +46,7 @@ mod sync {
     pub mod proactive_sync_plus;
     pub mod purgatory_fetch;
     pub mod reconnect_backoff;
+    pub mod repository_fixture;
     pub mod req_concurrency;
     pub mod stale_connect_result;
     pub mod tag_variations;
diff --git a/tests/sync/descendant_sync.rs b/tests/sync/descendant_sync.rs
index 672f55b..9a4d8ed 100644
--- a/tests/sync/descendant_sync.rs
+++ b/tests/sync/descendant_sync.rs
@@ -31,12 +31,14 @@ async fn historic_recursive_cap_is_enforced_and_reconstructed_after_restart() {
     let source = TestRelay::start_with_relay_max_subscriptions(4).await;
     let keys = Keys::generate();
     let repo_id = "restart-safe-recursive-cap";
-    let source_domains = [source.domain()];
+    let syncing_reservation = reserve_port();
+    let syncing_domain = format!("127.0.0.1:{}", syncing_reservation.port());
+    let source_domains = [source.domain(), syncing_domain];
     let source_refs = source_domains
         .iter()
         .map(String::as_str)
         .collect::<Vec<_>>();
-    let (_announcement, _source_git) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&source, &keys, &source_refs, repo_id).await;
     let source_client = TestClient::new(source.url(), keys.clone())
         .await
@@ -64,7 +66,7 @@ async fn historic_recursive_cap_is_enforced_and_reconstructed_after_restart() {
     let syncing_git_dir = tempfile::tempdir().expect("create syncing git dir");
     let syncing_data_dir = tempfile::tempdir().expect("create syncing relay data dir");
     let syncing = TestRelay::start_on_reservation_persistent_sync_with_recursive_limit(
-        reserve_port(),
+        syncing_reservation,
         None,
         true,
         syncing_git_dir.path().to_path_buf(),
@@ -72,10 +74,7 @@ async fn historic_recursive_cap_is_enforced_and_reconstructed_after_restart() {
         LIMIT,
     )
     .await;
-    let domains = [source.domain(), syncing.domain()];
-    let domain_refs = domains.iter().map(String::as_str).collect::<Vec<_>>();
-    let (_target_announcement, _target_git) =
-        setup_announcement_on_relay(&syncing, &keys, &domain_refs, repo_id).await;
+    repository.install(&syncing).await;
 
     for descendant in [&first, &second] {
         assert!(
@@ -154,12 +153,14 @@ async fn historic_sync_recovers_recursive_parent_only_descendants() {
 
     // Seed the source before the syncing relay exists so this exercises the
     // complete historic rotation, not ordinary live coverage.
-    let source_domains = [source.domain()];
+    let syncing_reservation = reserve_port();
+    let syncing_domain = format!("127.0.0.1:{}", syncing_reservation.port());
+    let source_domains = [source.domain(), syncing_domain];
     let source_refs = source_domains
         .iter()
         .map(String::as_str)
         .collect::<Vec<_>>();
-    let (_announcement, _source_git) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&source, &keys, &source_refs, repo_id).await;
     let source_client = TestClient::new(source.url(), keys.clone())
         .await
@@ -188,11 +189,9 @@ async fn historic_sync_recovers_recursive_parent_only_descendants() {
             .expect("seed source event");
     }
 
-    let syncing = TestRelay::start_with_sync(None).await;
-    let domains = [source.domain(), syncing.domain()];
-    let domain_refs = domains.iter().map(String::as_str).collect::<Vec<_>>();
-    let (_target_announcement, _target_git) =
-        setup_announcement_on_relay(&syncing, &keys, &domain_refs, repo_id).await;
+    let syncing =
+        TestRelay::start_on_reservation_with_options(syncing_reservation, None, false).await;
+    repository.install(&syncing).await;
 
     assert!(
         wait_for_event_on_relay(
@@ -235,12 +234,14 @@ async fn descendant_live_coverage_is_preferred_when_capacity_remains() {
     let source = TestRelay::start().await;
     let keys = Keys::generate();
     let repo_id = "live-descendant-coverage";
-    let source_domains = [source.domain()];
+    let syncing_reservation = reserve_port();
+    let syncing_domain = format!("127.0.0.1:{}", syncing_reservation.port());
+    let source_domains = [source.domain(), syncing_domain];
     let source_refs = source_domains
         .iter()
         .map(String::as_str)
         .collect::<Vec<_>>();
-    let (_announcement, _source_git) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&source, &keys, &source_refs, repo_id).await;
     let source_client = TestClient::new(source.url(), keys.clone())
         .await
@@ -252,11 +253,9 @@ async fn descendant_live_coverage_is_preferred_when_capacity_remains() {
     source_client.send_event(&issue).await.unwrap();
     source_client.send_event(&direct_reply).await.unwrap();
 
-    let syncing = TestRelay::start_with_sync(None).await;
-    let domains = [source.domain(), syncing.domain()];
-    let domain_refs = domains.iter().map(String::as_str).collect::<Vec<_>>();
-    let (_target_announcement, _target_git) =
-        setup_announcement_on_relay(&syncing, &keys, &domain_refs, repo_id).await;
+    let syncing =
+        TestRelay::start_on_reservation_with_options(syncing_reservation, None, false).await;
+    repository.install(&syncing).await;
     assert!(
         wait_for_log(
             &syncing.log_path(),
@@ -295,12 +294,14 @@ async fn historic_sync_recovers_address_tag_descendants() {
     let source = TestRelay::start_with_relay_max_subscriptions(4).await;
     let keys = Keys::generate();
     let repo_id = "addressable-descendant-history";
-    let source_domains = [source.domain()];
+    let syncing_reservation = reserve_port();
+    let syncing_domain = format!("127.0.0.1:{}", syncing_reservation.port());
+    let source_domains = [source.domain(), syncing_domain];
     let source_refs = source_domains
         .iter()
         .map(String::as_str)
         .collect::<Vec<_>>();
-    let (_announcement, _source_git) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&source, &keys, &source_refs, repo_id).await;
     let source_client = TestClient::new(source.url(), keys.clone())
         .await
@@ -334,11 +335,9 @@ async fn historic_sync_recovers_address_tag_descendants() {
         source_client.send_event(event).await.unwrap();
     }
 
-    let syncing = TestRelay::start_with_sync(None).await;
-    let domains = [source.domain(), syncing.domain()];
-    let domain_refs = domains.iter().map(String::as_str).collect::<Vec<_>>();
-    let (_target_announcement, _target_git) =
-        setup_announcement_on_relay(&syncing, &keys, &domain_refs, repo_id).await;
+    let syncing =
+        TestRelay::start_on_reservation_with_options(syncing_reservation, None, false).await;
+    repository.install(&syncing).await;
 
     for event in &address_only_children {
         assert!(
diff --git a/tests/sync/discovery.rs b/tests/sync/discovery.rs
index 84f549a..ef9315c 100644
--- a/tests/sync/discovery.rs
+++ b/tests/sync/discovery.rs
@@ -61,7 +61,7 @@ async fn test_discovers_layer3_via_layer2() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (announcement, _git_dir_a) =
+    let (announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     let announcement_id = announcement.id;
     println!(
@@ -102,8 +102,7 @@ async fn test_discovers_layer3_via_layer2() {
     client_a.disconnect().await;
 
     // 8. Set up announcement on relay_b (triggers discovery of relay_a)
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b (should trigger discovery of relay_a)");
 
     // 9/10. Verify the patch syncs to relay_b. The bounded poll on the synced
@@ -176,7 +175,7 @@ async fn test_relay_discovery_via_announcements_with_historic_sync() {
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
     let repo_id = "test-repo-chain";
 
-    let (announcement, _git_dir_a) =
+    let (announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     let announcement_id = announcement.id;
     println!(
@@ -207,8 +206,7 @@ async fn test_relay_discovery_via_announcements_with_historic_sync() {
     client_a.disconnect().await;
 
     // 6. Set up announcement on relay_b (triggers discovery of relay_a)
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b (should trigger discovery of relay_a)");
 
     // 7/8. Verify the Layer 2 event syncs to relay_b. The bounded poll on the
diff --git a/tests/sync/live_sync.rs b/tests/sync/live_sync.rs
index 4961b6c..fb9f1bd 100644
--- a/tests/sync/live_sync.rs
+++ b/tests/sync/live_sync.rs
@@ -299,12 +299,11 @@ async fn test_live_sync_layer2_events() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 5. Wait for discovery: events published before the syncing relay has
@@ -397,12 +396,11 @@ async fn test_live_sync_layer3_events() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has
@@ -573,10 +571,9 @@ async fn test_live_sync_event_ordering() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcements set up on both relays with git data");
 
     // 3. Wait for discovery: events published before the syncing relay has
diff --git a/tests/sync/metrics.rs b/tests/sync/metrics.rs
index 5bfe3d5..9ba34a8 100644
--- a/tests/sync/metrics.rs
+++ b/tests/sync/metrics.rs
@@ -243,7 +243,7 @@ async fn test_startup_sync_event_count() {
     let domains = [source_relay.domain(), syncing_relay.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (announcement, _git_dir_source) =
+    let (announcement, repository) =
         setup_announcement_on_relay(&source_relay, &keys, &domain_refs, repo_id).await;
     println!(
         "Announcement {} set up on source relay with git data",
@@ -286,8 +286,7 @@ async fn test_startup_sync_event_count() {
     source_client.disconnect().await;
 
     // 8. Set up announcement on SYNCING relay (triggers discovery of source relay)
-    let (_announcement_syncing, _git_dir_syncing) =
-        setup_announcement_on_relay(&syncing_relay, &keys, &domain_refs, repo_id).await;
+    repository.install(&syncing_relay).await;
     println!("Announcement set up on syncing relay (triggers discovery of source)");
 
     // 9. Wait for discovery + sync on the observable condition the test
diff --git a/tests/sync/repository_fixture.rs b/tests/sync/repository_fixture.rs
new file mode 100644
index 0000000..1d71d04
--- /dev/null
+++ b/tests/sync/repository_fixture.rs
@@ -0,0 +1,104 @@
+//! Reusing a repository fixture must not mint replaceable event revisions.
+use std::{collections::BTreeSet, time::Duration};
+
+use nostr_sdk::prelude::*;
+
+use crate::common::{reserve_port, setup_announcement_on_relay, TestRelay};
+
+async fn repository_event_ids(
+    relay: &TestRelay,
+    keys: &Keys,
+    identifier: &str,
+) -> BTreeSet<EventId> {
+    let client = Client::builder()
+        .authenticator(SignerAuthenticator::new(keys.clone()))
+        .build();
+    client.add_relay(relay.url()).await.unwrap();
+    client
+        .try_connect_relay(relay.url(), Duration::from_secs(5))
+        .await
+        .expect("connect to installed repository");
+    let events = client
+        .fetch_events(
+            Filter::new()
+                .author(keys.public_key())
+                .identifier(identifier)
+                .kinds([Kind::GitRepoAnnouncement, Kind::RepoState]),
+        )
+        .timeout(Duration::from_secs(5))
+        .await
+        .expect("fetch installed repository events");
+    client.disconnect().await;
+    events.iter().map(|event| event.id).collect()
+}
+
+async fn remote_head(relay: &TestRelay, keys: &Keys, identifier: &str) -> String {
+    let url = format!(
+        "http://{}/{}/{}.git",
+        relay.domain(),
+        keys.public_key().to_bech32().unwrap(),
+        identifier
+    );
+    let output = tokio::process::Command::from(grasp_audit::git_command())
+        .args(["ls-remote", &url, "refs/heads/main"])
+        .kill_on_drop(true)
+        .output()
+        .await
+        .expect("query repository ref");
+    assert!(output.status.success());
+    String::from_utf8(output.stdout)
+        .unwrap()
+        .split_whitespace()
+        .next()
+        .expect("main ref exists")
+        .to_string()
+}
+
+#[tokio::test]
+async fn reinstalling_shared_repository_preserves_event_ids_and_git_refs() {
+    tokio::time::timeout(Duration::from_secs(120), async {
+        let source = TestRelay::start().await;
+        let target_reservation = reserve_port();
+        let target_domain = format!("127.0.0.1:{}", target_reservation.port());
+        let domains = [source.domain(), target_domain];
+        let refs = domains.iter().map(String::as_str).collect::<Vec<_>>();
+        let keys = Keys::generate();
+        let identifier = "shared-fixture-identity";
+        let (announcement, repository) =
+            setup_announcement_on_relay(&source, &keys, &refs, identifier).await;
+        let original_ids = repository_event_ids(&source, &keys, identifier).await;
+        assert_eq!(
+            original_ids.len(),
+            2,
+            "announcement and state must both exist"
+        );
+        assert!(original_ids.contains(&announcement.id));
+        let original_head = remote_head(&source, &keys, identifier).await;
+
+        // Start the peer only after seeding the source, matching historic sync.
+        let target =
+            TestRelay::start_on_reservation_with_options(target_reservation, None, false).await;
+        repository.install(&target).await;
+        assert_eq!(
+            repository_event_ids(&target, &keys, identifier).await,
+            original_ids
+        );
+        assert_eq!(remote_head(&target, &keys, identifier).await, original_head);
+
+        // Reinstall after sync has seen the same address. There must be no new
+        // event IDs, timestamps or competing Git state in either direction.
+        repository.install(&source).await;
+        repository.install(&target).await;
+        for relay in [&source, &target] {
+            assert_eq!(
+                repository_event_ids(relay, &keys, identifier).await,
+                original_ids
+            );
+            assert_eq!(remote_head(relay, &keys, identifier).await, original_head);
+        }
+        target.stop().await;
+        source.stop().await;
+    })
+    .await
+    .expect("shared repository setup and reinstalls must complete");
+}
diff --git a/tests/sync/tag_variations.rs b/tests/sync/tag_variations.rs
index 5fb764d..1e43ac5 100644
--- a/tests/sync/tag_variations.rs
+++ b/tests/sync/tag_variations.rs
@@ -61,12 +61,11 @@ async fn test_layer2_sync_with_lowercase_a_tag() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has
@@ -152,12 +151,11 @@ async fn test_layer2_sync_with_uppercase_a_tag() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has
@@ -245,12 +243,11 @@ async fn test_layer2_sync_with_q_tag() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has
@@ -344,12 +341,11 @@ async fn test_layer3_sync_with_lowercase_e_tag() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has
@@ -460,12 +456,11 @@ async fn test_layer3_sync_with_uppercase_e_tag() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has
@@ -580,12 +575,11 @@ async fn test_layer3_sync_with_q_tag() {
     let domains = [relay_a.domain(), relay_b.domain()];
     let domain_refs: Vec<&str> = domains.iter().map(|s| s.as_str()).collect();
 
-    let (_announcement, _git_dir_a) =
+    let (_announcement, repository) =
         setup_announcement_on_relay(&relay_a, &keys, &domain_refs, repo_id).await;
     println!("Announcement set up on relay_a with git data");
 
-    let (_announcement_b, _git_dir_b) =
-        setup_announcement_on_relay(&relay_b, &keys, &domain_refs, repo_id).await;
+    repository.install(&relay_b).await;
     println!("Announcement set up on relay_b with git data (triggers discovery)");
 
     // 3. Wait for discovery: events published before the syncing relay has