diff --git a/jam-ui/src/store/features/sessionChatSlice.js b/jam-ui/src/store/features/sessionChatSlice.js
index 414d6edff..2738aa903 100644
--- a/jam-ui/src/store/features/sessionChatSlice.js
+++ b/jam-ui/src/store/features/sessionChatSlice.js
@@ -185,9 +185,18 @@ const sessionChatSlice = createSlice({
         state.messagesByChannel[channel] = [];
       }
 
-      // Deduplicate by msg_id
-      const exists = state.messagesByChannel[channel].some(m => m.id === message.id);
-      if (exists) return;
+      // Deduplicate by msg_id and attachmentId (for attachment messages)
+      const existsById = state.messagesByChannel[channel].some(m => m.id === message.id);
+      if (existsById) return;
+
+      // For attachment messages, also check by attachmentId to avoid duplicates
+      // from optimistic updates that use different ID format
+      if (message.attachmentId) {
+        const existsByAttachmentId = state.messagesByChannel[channel].some(
+          m => m.attachmentId && m.attachmentId === message.attachmentId
+        );
+        if (existsByAttachmentId) return;
+      }
 
       // Add message
       state.messagesByChannel[channel].push(message);
@@ -335,9 +344,23 @@ const sessionChatSlice = createSlice({
           attachmentSize: null  // Not available in REST API response (only in WebSocket)
         }));
 
-        // Deduplicate messages
+        // Deduplicate messages by ID and by attachmentId (for attachment messages)
+        // This handles the case where optimistic attachment message uses 'attachment-{id}' format
+        // but the REST API returns the chat message ID
         const existingIds = new Set(state.messagesByChannel[channel].map(m => m.id));
-        const newMessages = transformedMessages.filter(m => !existingIds.has(m.id));
+        const existingAttachmentIds = new Set(
+          state.messagesByChannel[channel]
+            .filter(m => m.attachmentId)
+            .map(m => m.attachmentId)
+        );
+        const newMessages = transformedMessages.filter(m => {
+          // Check by message ID first
+          if (existingIds.has(m.id)) return false;
+          // For attachment messages, also check by attachmentId to avoid duplicates
+          // from optimistic updates that use different ID format
+          if (m.attachmentId && existingAttachmentIds.has(m.attachmentId)) return false;
+          return true;
+        });
 
         // Prepend new messages (oldest first for pagination)
         state.messagesByChannel[channel] = [...newMessages, ...state.messagesByChannel[channel]];