diff --git a/jam-ui/src/store/features/sessionChatSlice.js b/jam-ui/src/store/features/sessionChatSlice.js
index 2738aa903..a71142e11 100644
--- a/jam-ui/src/store/features/sessionChatSlice.js
+++ b/jam-ui/src/store/features/sessionChatSlice.js
@@ -468,9 +468,13 @@ const sessionChatSlice = createSlice({
           // Use music_notation.id as part of message id to avoid collision with real msg_id
           const messageId = `attachment-${notation.id}`;
 
-          // Check for duplicates (in case WebSocket somehow delivers to sender)
-          const exists = state.messagesByChannel[channel].some(m => m.id === messageId);
-          if (!exists) {
+          // Check for duplicates by both message ID AND attachmentId
+          // WebSocket may deliver message before this handler runs with different ID format
+          const existsById = state.messagesByChannel[channel].some(m => m.id === messageId);
+          const existsByAttachmentId = state.messagesByChannel[channel].some(
+            m => m.attachmentId && m.attachmentId === notation.id
+          );
+          if (!existsById && !existsByAttachmentId) {
             const purpose = attachmentType === 'audio' ? 'Audio File' : 'Notation File';
             const message = {
               id: messageId,