fix(chat): deduplicate message ids (#810)

app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt

@@ -68,6 +68,16 @@ class ChatViewModelTest {
     /** Counter used to generate unique WS request IDs. */
     private var reqCount = 0
 
+    @Test
+    fun mergeTranscriptWithLive_collapsesDuplicateIdsKeepingLatestMessage() {
+        val stale = ChatMessage(id = "duplicate-id", role = MessageRole.ASSISTANT, content = "stale")
+        val latest = stale.copy(content = "latest")
+
+        val merged = mergeTranscriptWithLive(emptyList(), listOf(stale, latest))
+
+        assertEquals(listOf(latest), merged)
+    }
+
     @Before
     fun setUp() {
         Dispatchers.setMain(testDispatcher)

app/src/test/java/com/m57/hermescontrol/ui/chat/ChatWsEventReducerTest.kt

@@ -377,6 +377,34 @@ class ChatWsEventReducerTest {
         assertEquals("", persistEffect?.message?.reasoningText)
     }
 
+    @Test
+    fun testMessageComplete_replacesExistingStreamingMessageWithSameId() {
+        val streaming =
+            ChatMessage(
+                id = "037cae0c-494e-4a36-862f-3f3e8a235950",
+                role = MessageRole.ASSISTANT,
+                content = "partial",
+                isStreaming = true,
+            )
+        val result =
+            ChatWsEventReducer.reduce(
+                state =
+                    ChatUiState(
+                        messages =
+                            listOf(
+                                streaming.copy(content = "stale", isStreaming = false),
+                                streaming.copy(isStreaming = false),
+                            ),
+                    ),
+                streamingState = StreamingState(streamingMessage = streaming),
+                event = WsEvent.MessageComplete(text = "complete", sessionId = "session-1"),
+                currentSessionId = "session-1",
+            )
+
+        assertEquals(1, result.state.messages.size)
+        assertEquals("complete", result.state.messages.single().content)
+    }
+
     @Test
     fun testMessageStart_prunesCompletedSubagents() {
         val completedSubagent =