Deduplicate merged trace messages and fix double-count in context

- Add dedup helper in computeMergedMessages to remove duplicate messages
  by ID across all merge strategies (e.g. A-B-C + A-D-C → A-B-D-C)
- Fix getActiveContext adding merged trace messages twice (from both
  outgoingTraces and mergedTraces)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 30 insertions, 18 deletions

diff --git a/frontend/src/store/flowStore.ts b/frontend/src/store/flowStore.ts
index 72298b8..665030b 100644
--- a/frontend/src/store/flowStore.ts
+++ b/frontend/src/store/flowStore.ts
@@ -1070,9 +1070,9 @@ const useFlowStore = create<FlowState>((set, get) => {
       }
     });
     
-    // Check merged traces
-    const activeMerged = (node.data.mergedTraces || []).filter((m: MergedTrace) => 
-      activeIds.includes(m.id)
+    // Check merged traces (skip if already added from outgoingTraces above)
+    const activeMerged = (node.data.mergedTraces || []).filter((m: MergedTrace) =>
+      activeIds.includes(m.id) && !tracesById.has(m.id)
     );
     activeMerged.forEach((m: MergedTrace) => {
       contextMessages.push(...m.messages);
@@ -1898,6 +1898,18 @@ const useFlowStore = create<FlowState>((set, get) => {
       return allMessages;
     };
 
+    // Deduplicate messages by ID, keeping first occurrence
+    // This handles cases like A-B-C + A-D-C merged at C, where A's messages appear in both traces
+    const dedup = (messages: Message[]): Message[] => {
+      const seen = new Set<string>();
+      return messages.filter(msg => {
+        if (!msg.id) return true; // keep messages without ID
+        if (seen.has(msg.id)) return false;
+        seen.add(msg.id);
+        return true;
+      });
+    };
+
     switch (strategy) {
       case 'query_time': {
         // Sort by query time, keeping Q-A pairs together
@@ -1918,41 +1930,41 @@ const useFlowStore = create<FlowState>((set, get) => {
         });
         
         pairs.sort((a, b) => a.time - b.time);
-        
+
         const result: Message[] = [];
         pairs.forEach(pair => {
           if (pair.user) result.push(pair.user);
           if (pair.assistant) result.push(pair.assistant);
         });
-        return result;
+        return dedup(result);
       }
 
       case 'response_time': {
         // Sort by response time, keeping Q-A pairs together
         const pairs: { user: Message | null; assistant: Message | null; time: number; trace: Trace }[] = [];
-        
+
         sourceTraces.forEach(trace => {
           for (let i = 0; i < trace.messages.length; i += 2) {
             const user = trace.messages[i];
             const assistant = trace.messages[i + 1] || null;
             const time = getMessageTimestamp(assistant || user, 'response');
-            pairs.push({ 
-              user: user ? tagMessage(user, trace) : null, 
-              assistant: assistant ? tagMessage(assistant, trace) : null, 
+            pairs.push({
+              user: user ? tagMessage(user, trace) : null,
+              assistant: assistant ? tagMessage(assistant, trace) : null,
               time,
               trace
             });
           }
         });
-        
+
         pairs.sort((a, b) => a.time - b.time);
-        
+
         const result: Message[] = [];
         pairs.forEach(pair => {
           if (pair.user) result.push(pair.user);
           if (pair.assistant) result.push(pair.assistant);
         });
-        return result;
+        return dedup(result);
       }
 
       case 'trace_order': {
@@ -1963,7 +1975,7 @@ const useFlowStore = create<FlowState>((set, get) => {
             result.push(tagMessage(msg, trace));
           });
         });
-        return result;
+        return dedup(result);
       }
 
       case 'grouped': {
@@ -1974,21 +1986,21 @@ const useFlowStore = create<FlowState>((set, get) => {
             result.push(tagMessage(msg, trace));
           });
         });
-        return result;
+        return dedup(result);
       }
 
       case 'interleaved': {
         // True interleaving - sort ALL messages by their actual time
         const allMessages = getAllMessagesWithTime();
-        
+
         // Sort by the earlier of query/response time for each message
         allMessages.sort((a, b) => {
           const aTime = a.msg.role === 'user' ? a.queryTime : a.responseTime;
           const bTime = b.msg.role === 'user' ? b.queryTime : b.responseTime;
           return aTime - bTime;
         });
-        
-        return allMessages.map(m => m.msg);
+
+        return dedup(allMessages.map(m => m.msg));
       }
 
       case 'summary': {
@@ -2000,7 +2012,7 @@ const useFlowStore = create<FlowState>((set, get) => {
             result.push(tagMessage(msg, trace));
           });
         });
-        return result;
+        return dedup(result);
       }
 
       default: