diff --git a/geode/src/test/kotlin/com/vitorpamplona/geode/KtorRelayTest.kt b/geode/src/test/kotlin/com/vitorpamplona/geode/KtorRelayTest.kt
index b79f81bf51..ea1a1352bd 100644
--- a/geode/src/test/kotlin/com/vitorpamplona/geode/KtorRelayTest.kt
+++ b/geode/src/test/kotlin/com/vitorpamplona/geode/KtorRelayTest.kt
@@ -311,14 +311,14 @@ class KtorRelayTest {
                 )
             assertEquals(true, ok, "successful insert must round-trip OK true on the wire")
 
-            // Duplicate insert returns OK false; this also exercises the
-            // "non-empty message" branch of the serializer.
+            // Duplicate insert returns OK true with a `duplicate:` message (NIP-01);
+            // this also exercises the "non-empty message" branch of the serializer.
             val ok2 =
                 client.publishAndConfirm(
                     event = event,
                     relayList = setOf(server.url.normalizeRelayUrl()),
                 )
-            assertEquals(false, ok2, "duplicate insert must round-trip OK false")
+            assertEquals(true, ok2, "duplicate insert must round-trip OK true (NIP-01 duplicate:)")
         }
 
     /**
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/RelaySession.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/RelaySession.kt
index 9520dd9848..33a01e0e92 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/RelaySession.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/RelaySession.kt
@@ -41,6 +41,7 @@ import com.vitorpamplona.quartz.nip01Core.relay.server.policies.IRelayPolicy
 import com.vitorpamplona.quartz.nip01Core.relay.server.policies.PolicyResult
 import com.vitorpamplona.quartz.nip01Core.store.IEventStore
 import com.vitorpamplona.quartz.nip01Core.store.RawEvent
+import com.vitorpamplona.quartz.nip01Core.store.RejectionReason
 import com.vitorpamplona.quartz.nip77Negentropy.NegCloseCmd
 import com.vitorpamplona.quartz.nip77Negentropy.NegMsgCmd
 import com.vitorpamplona.quartz.nip77Negentropy.NegOpenCmd
@@ -212,7 +213,13 @@ class RelaySession(
                     }
 
                     is IEventStore.InsertOutcome.Rejected -> {
-                        send(OkMessage(cmd.event.id, false, outcome.reason))
+                        // NIP-01: an event the relay already holds is acknowledged with
+                        // `OK true` and the `duplicate:` prefix. Every real client
+                        // (amethyst's outbox included) resends an event whose OK has not
+                        // landed yet, and treats OK false as a rejection to surface —
+                        // so answering false here turns a routine resend into an error.
+                        val duplicate = outcome.reason.startsWith(RejectionReason.PREFIX_DUPLICATE)
+                        send(OkMessage(cmd.event.id, duplicate, outcome.reason))
                     }
 
                     is IEventStore.InsertOutcome.Failed -> {
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/RejectionReason.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/RejectionReason.kt
index 4e14cb2550..90e416f230 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/RejectionReason.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/RejectionReason.kt
@@ -44,6 +44,9 @@ object RejectionReason {
      */
     const val PREFIX_REPLACED = "replaced:"
 
+    /** NIP-01 prefix for "already have this event" — answered with `OK true`, not false. */
+    const val PREFIX_DUPLICATE = "duplicate:"
+
     // The standard store reasons.
     const val DUPLICATE = "duplicate: already have this event"
     const val EXPIRED = "blocked: Cannot insert an expired event"
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/sqlite/SQLiteEventStore.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/sqlite/SQLiteEventStore.kt
index 1b3643543c..080d26f4c4 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/sqlite/SQLiteEventStore.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/store/sqlite/SQLiteEventStore.kt
@@ -62,6 +62,8 @@ class SQLiteEventStore(
     val extraPragmas: List<String> = emptyList(),
 ) {
     companion object {
+        /** SQLite's message for the unique index on `event_headers (id)`. */
+        private const val DUPLICATE_ID_CONSTRAINT = "UNIQUE constraint failed: event_headers.id"
         const val DATABASE_VERSION = 5
     }
 
@@ -526,6 +528,14 @@ class SQLiteEventStore(
      */
     private fun classifyRowError(e: Throwable): IEventStore.InsertOutcome {
         val message = e.message ?: e::class.simpleName ?: RejectionReason.INSERT_FAILED
+        // A second copy of an event the store already holds trips the unique index on
+        // event_headers.id. That is not a refusal of the event but a statement that it
+        // is already here, and NIP-01 has a dedicated answer for it (`OK true` with the
+        // `duplicate:` prefix) — so name it, instead of leaking SQLite's constraint text
+        // for the session to turn into a rejection the client then retries or reports.
+        if (message.contains(DUPLICATE_ID_CONSTRAINT)) {
+            return IEventStore.InsertOutcome.Rejected(RejectionReason.DUPLICATE)
+        }
         val refusal =
             message.contains("blocked:") ||
                 message.contains("duplicate:") ||
diff --git a/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/NostrServerTest.kt b/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/NostrServerTest.kt
index 7171a9e4a0..39d27b15fc 100644
--- a/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/NostrServerTest.kt
+++ b/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/relay/server/NostrServerTest.kt
@@ -120,8 +120,13 @@ class NostrServerTest {
             server.close()
         }
 
+    /**
+     * NIP-01: `["OK", <id>, true, "duplicate: already have this event"]`. A client
+     * resends any event whose OK has not landed, so a duplicate must read as
+     * success — OK false would make every such resend look like a rejection.
+     */
     @Test
-    fun duplicateEventReturnsOkFalse() =
+    fun duplicateEventReturnsOkTrueWithDuplicatePrefix() =
         runTest {
             val dispatcher = UnconfinedTestDispatcher(testScheduler)
             val store = EventStore(null)
@@ -138,7 +143,8 @@ class NostrServerTest {
             val okMessages = collector.rawMessagesContaining("OK")
             assertEquals(2, okMessages.size)
             assertTrue(okMessages[0].contains(",true,"))
-            assertTrue(okMessages[1].contains(",false,"))
+            assertTrue(okMessages[1].contains(",true,"))
+            assertTrue(okMessages[1].contains("duplicate:"))
 
             server.close()
         }