diff --git a/Core/Cleipnir.ResilientFunctions/Queuing/QueueManager.cs b/Core/Cleipnir.ResilientFunctions/Queuing/QueueManager.cs
index 74ef9eaa..eaf8b7bc 100644
--- a/Core/Cleipnir.ResilientFunctions/Queuing/QueueManager.cs
+++ b/Core/Cleipnir.ResilientFunctions/Queuing/QueueManager.cs
@@ -227,7 +227,11 @@ private async Task ProcessMessages(IReadOnlyList<StoredMessage> messages)
 
                 if (idempotencyKey != null && !_idempotencyKeys.Add(idempotencyKey, position))
                 {
-                    await _messageClearer.Clear([position]);
+                    lock (_lock)
+                    {
+                        _deliveredPositions.Add(position);
+                        _effect.FlushlessUpsert(DeliveredPositionsId, _deliveredPositions.ToList(), alias: null);
+                    }
                     continue;
                 }