diff --git a/src/domain/session/room/timeline/ReactionsViewModel.js b/src/domain/session/room/timeline/ReactionsViewModel.js
index 3fd9f15f..a9ac4f21 100644
--- a/src/domain/session/room/timeline/ReactionsViewModel.js
+++ b/src/domain/session/room/timeline/ReactionsViewModel.js
@@ -247,8 +247,8 @@ export function tests() {
                 storage.storeNames.timelineFragments
             ]);
             txn.timelineFragments.add({id: 1, roomId});
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 2, event: messageEvent, roomId});
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 3, event: myReactionEvent, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 2, event: messageEvent, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 3, event: myReactionEvent, roomId});
             await relationWriter.writeRelation(myReactionEntry, txn, new NullLogItem());
             await txn.complete();
             // 2. setup queue & timeline
@@ -309,7 +309,7 @@ export function tests() {
                 storage.storeNames.timelineFragments
             ]);
             txn.timelineFragments.add({id: 1, roomId});
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 2, event: messageEvent, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 2, event: messageEvent, roomId});
             await txn.complete();
             // 2. setup queue & timeline
             const queue = new SendQueue({roomId, storage, hsApi: new MockHomeServer().api});
diff --git a/src/matrix/room/timeline/Timeline.js b/src/matrix/room/timeline/Timeline.js
index 54eabf96..1677a840 100644
--- a/src/matrix/room/timeline/Timeline.js
+++ b/src/matrix/room/timeline/Timeline.js
@@ -447,7 +447,7 @@ export function tests() {
             // 1. put event and reaction into storage
             const storage = await createMockStorage();
             const txn = await storage.readWriteTxn([storage.storeNames.timelineEvents, storage.storeNames.timelineRelations]);
-            txn.timelineEvents.insert({
+            txn.timelineEvents.tryInsert({
                 event: withContent(createAnnotation(messageId, "👋"), createEvent("m.reaction", reactionId, bob)),
                 fragmentId: 1, eventIndex: 1, roomId
             });
@@ -543,7 +543,7 @@ export function tests() {
             // 1. put reaction in storage
             const storage = await createMockStorage();
             const txn = await storage.readWriteTxn([storage.storeNames.timelineEvents, storage.storeNames.timelineRelations]);
-            txn.timelineEvents.insert({
+            txn.timelineEvents.tryInsert({
                 event: withContent(createAnnotation(messageId, "👋"), createEvent("m.reaction", reactionId, bob)),
                 fragmentId: 1, eventIndex: 3, roomId
             });
diff --git a/src/matrix/room/timeline/persistence/GapWriter.js b/src/matrix/room/timeline/persistence/GapWriter.js
index e133d713..be14922a 100644
--- a/src/matrix/room/timeline/persistence/GapWriter.js
+++ b/src/matrix/room/timeline/persistence/GapWriter.js
@@ -124,9 +124,12 @@ export class GapWriter {
             if (updatedRelationTargetEntries) {
                 updatedEntries.push(...updatedRelationTargetEntries);
             }
-            txn.timelineEvents.insert(eventStorageEntry);
-            const eventEntry = new EventEntry(eventStorageEntry, this._fragmentIdComparer);
-            directionalAppend(entries, eventEntry, direction);
+            if (await txn.timelineEvents.tryInsert(eventStorageEntry)) {
+                const eventEntry = new EventEntry(eventStorageEntry, this._fragmentIdComparer);
+                directionalAppend(entries, eventEntry, direction);
+            } else {
+                log.log({l: `could not write event`, id: event.event_id}, log.level.Warn);
+            }
         }
         return {entries, updatedEntries};
     }
diff --git a/src/matrix/room/timeline/persistence/RelationWriter.js b/src/matrix/room/timeline/persistence/RelationWriter.js
index 4116b775..d8619cf9 100644
--- a/src/matrix/room/timeline/persistence/RelationWriter.js
+++ b/src/matrix/room/timeline/persistence/RelationWriter.js
@@ -275,7 +275,7 @@ export function tests() {
 
             const storage = await createMockStorage();
             const txn = await storage.readWriteTxn([storage.storeNames.timelineEvents, storage.storeNames.timelineRelations]);
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 2, event, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 2, event, roomId});
             const updatedEntries = await relationWriter.writeRelation(redactionEntry, txn, new NullLogItem());
             await txn.complete();
 
@@ -300,7 +300,7 @@ export function tests() {
 
             const storage = await createMockStorage();
             const txn = await storage.readWriteTxn([storage.storeNames.timelineEvents, storage.storeNames.timelineRelations]);
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 2, event, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 2, event, roomId});
             const updatedEntries = await relationWriter.writeRelation(reactionEntry, txn, new NullLogItem());
             await txn.complete();
 
@@ -329,7 +329,7 @@ export function tests() {
 
             const storage = await createMockStorage();
             const txn = await storage.readWriteTxn([storage.storeNames.timelineEvents, storage.storeNames.timelineRelations]);
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 2, event, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 2, event, roomId});
             await relationWriter.writeRelation(reaction1Entry, txn, new NullLogItem());
             const updatedEntries = await relationWriter.writeRelation(reaction2Entry, txn, new NullLogItem());
             await txn.complete();
@@ -358,10 +358,10 @@ export function tests() {
 
             const storage = await createMockStorage();
             const txn = await storage.readWriteTxn([storage.storeNames.timelineEvents, storage.storeNames.timelineRelations]);
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 2, event, roomId});
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 3, event: myReaction, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 2, event, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 3, event: myReaction, roomId});
             await relationWriter.writeRelation(myReactionEntry, txn, new NullLogItem());
-            txn.timelineEvents.insert({fragmentId: 1, eventIndex: 4, event: bobReaction, roomId});
+            txn.timelineEvents.tryInsert({fragmentId: 1, eventIndex: 4, event: bobReaction, roomId});
             await relationWriter.writeRelation(bobReactionEntry, txn, new NullLogItem());
             const updatedEntries = await relationWriter.writeRelation(myReactionRedactionEntry, txn, new NullLogItem());
             await txn.complete();
diff --git a/src/matrix/room/timeline/persistence/SyncWriter.js b/src/matrix/room/timeline/persistence/SyncWriter.js
index 07326225..36d3c913 100644
--- a/src/matrix/room/timeline/persistence/SyncWriter.js
+++ b/src/matrix/room/timeline/persistence/SyncWriter.js
@@ -162,7 +162,11 @@ export class SyncWriter {
                     storageEntry.displayName = member.displayName;
                     storageEntry.avatarUrl = member.avatarUrl;
                 }
-                txn.timelineEvents.insert(storageEntry, log);
+                const couldInsert = await txn.timelineEvents.tryInsert(storageEntry, log);
+                if (!couldInsert) {
+                    log.log({l: `could not write event, likely #504`, id: event.event_id}, log.level.Warn);
+                    continue;
+                }
                 const entry = new EventEntry(storageEntry, this._fragmentIdComparer);
                 entries.push(entry);
                 const updatedRelationTargetEntries = await this._relationWriter.writeRelation(entry, txn, log);
diff --git a/src/matrix/storage/idb/stores/TimelineEventStore.ts b/src/matrix/storage/idb/stores/TimelineEventStore.ts
index 535b8f7e..3d27e8a7 100644
--- a/src/matrix/storage/idb/stores/TimelineEventStore.ts
+++ b/src/matrix/storage/idb/stores/TimelineEventStore.ts
@@ -261,15 +261,17 @@ export class TimelineEventStore {
         return firstFoundKey && decodeEventIdKey(firstFoundKey).eventId;
     }
 
-    /** Inserts a new entry into the store. The combination of roomId and eventKey should not exist yet, or an error is thrown.
-     *  @param entry the entry to insert
-     *  @return nothing. To wait for the operation to finish, await the transaction it's part of.
-     *  @throws {StorageError} ...
+    /** Inserts a new entry into the store.
+     * 
+     * If the event already exists in the store (either the eventKey or the event id
+     * are already known for the given roomId), this operation has no effect.
+     * 
+     * Returns if the event was not yet known and the entry was written.
      */
-    insert(entry: TimelineEventEntry, log: LogItem): void {
+    tryInsert(entry: TimelineEventEntry, log: LogItem): Promise<boolean> {
         (entry as TimelineEventStorageEntry).key = encodeKey(entry.roomId, entry.fragmentId, entry.eventIndex);
         (entry as TimelineEventStorageEntry).eventIdKey = encodeEventIdKey(entry.roomId, entry.event.event_id);
-        this._timelineStore.add(entry as TimelineEventStorageEntry, log);
+        return this._timelineStore.tryAdd(entry as TimelineEventStorageEntry, log);
     }
 
     /** Updates the entry into the store with the given [roomId, eventKey] combination.