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

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -154,25 +154,27 @@ internal fun mergeTranscriptWithLive(
     restMessages: List<ChatMessage>,
     currentMessages: List<ChatMessage>,
 ): List<ChatMessage> {
+    val dedupedRest = restMessages.dedupeById()
+    val dedupedCurrent = currentMessages.dedupeById()
     // User rows can carry live/cache-only metadata (for example whether the
     // bubble continues the active turn). Keep that richer copy when REST
     // returns the same logical row.
-    val currentUsers = currentMessages.filter { it.role == MessageRole.USER }.toMutableList()
+    val currentUsers = dedupedCurrent.filter { it.role == MessageRole.USER }.toMutableList()
     val mergedRest =
-        restMessages.map { rest ->
+        dedupedRest.map { rest ->
             if (rest.role != MessageRole.USER) return@map rest
             val matchIndex =
                 currentUsers.indexOfFirst { it.id == rest.id }.takeIf { it >= 0 }
                     ?: currentUsers.indexOfFirst { sameLogicalMessage(rest, it) }
             if (matchIndex >= 0) currentUsers.removeAt(matchIndex) else rest
         }
-    val restIds = restMessages.map { it.id }.toSet()
+    val restIds = dedupedRest.map { it.id }.toSet()
     val liveTail =
-        currentMessages.filter { old ->
+        dedupedCurrent.filter { old ->
-            old.id !in restIds && restMessages.none { sameLogicalMessage(it, old) }
+            old.id !in restIds && mergedRest.none { sameLogicalMessage(it, old) }
         }
-    if (liveTail.isEmpty()) return mergedRest
+    if (liveTail.isEmpty()) return mergedRest.dedupeById()
-    return (mergedRest + liveTail).sortedBy { it.timestamp }
+    return (mergedRest + liveTail).dedupeById().sortedBy { it.timestamp }
 }
 
 /** Merge a REST page without matching it against already-settled transcript rows. */
@@ -186,8 +188,10 @@ internal fun mergeIncrementalTranscriptPage(
         currentMessages.indexOfLast { message ->
             serverMessageIndex(message.id, sessionId)?.let { it < offset } == true
         }
-    return currentMessages.take(settledEnd + 1) +
+    return (
-        mergeTranscriptWithLive(restMessages, currentMessages.drop(settledEnd + 1))
+        currentMessages.take(settledEnd + 1) +
+            mergeTranscriptWithLive(restMessages, currentMessages.drop(settledEnd + 1))
+    ).dedupeById()
 }
 
 private fun serverMessageIndex(
@@ -195,6 +199,8 @@ private fun serverMessageIndex(
     sessionId: String,
 ): Int? = id.removePrefix("rest-$sessionId-").takeIf { it != id }?.toIntOrNull()
 
+internal fun List<ChatMessage>.dedupeById(): List<ChatMessage> = associateBy { it.id }.values.toList()
+
 data class ChatUiState(
     val messages: List<ChatMessage> = emptyList(),
     val currentSessionId: String? = null,

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatWsEventReducer.kt

@@ -8,6 +8,10 @@ import kotlinx.serialization.json.JsonArray
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.JsonPrimitive
 
+private fun List<ChatMessage>.upsertById(message: ChatMessage): List<ChatMessage> {
+    return (this + message).dedupeById()
+}
+
 /**
  * Pure state reducer for WebSocket events.
  *
@@ -168,7 +172,7 @@ object ChatWsEventReducer {
                 val finalized = streamingState.streamingMessage.copy(isStreaming = false)
                 orphan = finalized
                 state.copy(
-                    messages = state.messages + finalized,
+                    messages = state.messages.upsertById(finalized),
                     isAgentTyping = true,
                     subagentIndicators = cleanedSubagents,
                 )
@@ -319,7 +323,7 @@ object ChatWsEventReducer {
         return ReducerResult(
             state =
                 state.copy(
-                    messages = state.messages + msg,
+                    messages = state.messages.upsertById(msg),
                     isAgentTyping = false,
                 ),
             effects = effects,
@@ -358,7 +362,7 @@ object ChatWsEventReducer {
         return ReducerResult(
             state =
                 state.copy(
-                    messages = state.messages + msg,
+                    messages = state.messages.upsertById(msg),
                     isAgentTyping = false,
                 ),
             effects = effects,
@@ -401,7 +405,7 @@ object ChatWsEventReducer {
                     )
                 orphanToPersist = finalized
                 state.copy(
-                    messages = state.messages + finalized,
+                    messages = state.messages.upsertById(finalized),
                 )
             } else {
                 state

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 =