fix: message ordering and duplicate messages

Backend:
- Composite ORDER BY (created_at DESC, iteration ASC, id ASC) to fix
  non-deterministic ordering when multiple messages share same second
- Composite key cursor pagination to avoid missing messages at boundary

Android:
- Fix message duplication: guard loadMoreHistory from firing before
  initial scroll completes (isAtTop race → fetchOlderMessages →
  cacheMessagesFromApi with different IDs → Room duplicates)
- Add distinctBy(id) at all message assignment points in ViewModel
- cacheMessagesFromApi: skip insert if same ID or same content exists
- Add relative timestamp display on message bubbles

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

android/app/src/main/java/com/tiangong/aiagent/data/local/MessageDao.kt

@@ -12,7 +12,7 @@ interface MessageDao {
     @Insert(onConflict = OnConflictStrategy.REPLACE)
     suspend fun insertAll(messages: List<MessageEntity>)
 
-    @Query("SELECT * FROM messages WHERE conversationId = :conversationId ORDER BY createdAt ASC")
+    @Query("SELECT * FROM messages WHERE conversationId = :conversationId ORDER BY createdAt ASC, id ASC")
     fun getMessagesByConversation(conversationId: String): Flow<List<MessageEntity>>
 
     @Query("SELECT * FROM messages WHERE conversationId = :conversationId ORDER BY createdAt ASC LIMIT 1")
@@ -24,9 +24,15 @@ interface MessageDao {
     @Query("DELETE FROM messages")
     suspend fun deleteAll()
 
-    @Query("SELECT * FROM messages WHERE conversationId = :conversationId AND createdAt < :beforeTimestamp ORDER BY createdAt DESC LIMIT :limit")
+    @Query("SELECT * FROM messages WHERE conversationId = :conversationId AND createdAt < :beforeTimestamp ORDER BY createdAt DESC, id DESC LIMIT :limit")
     suspend fun getMessagesBeforeTimestamp(conversationId: String, beforeTimestamp: Long, limit: Int): List<MessageEntity>
 
     @Query("SELECT COUNT(*) FROM messages WHERE conversationId = :conversationId")
     suspend fun getMessageCount(conversationId: String): Int
+
+    @Query("SELECT * FROM messages WHERE id = :id LIMIT 1")
+    suspend fun getMessageById(id: String): MessageEntity?
+
+    @Query("SELECT COUNT(*) FROM messages WHERE conversationId = :conversationId AND role = :role AND content = :content")
+    suspend fun countByConvRoleContent(conversationId: String, role: String, content: String): Int
 }

android/app/src/main/java/com/tiangong/aiagent/data/repository/ChatRepository.kt

@@ -329,6 +329,10 @@ class ChatRepository @Inject constructor(
     private suspend fun cacheMessagesFromApi(dtos: List<MessageItemDto>, sessionId: String, agentId: String?) {
         val dao = database.messageDao()
         for (dto in dtos) {
+            // Skip if already cached (same server ID)
+            if (dao.getMessageById(dto.id) != null) continue
+            // Skip if same content already saved locally (prevents local-ID vs server-ID duplicates)
+            if (dao.countByConvRoleContent(sessionId, dto.role, dto.content ?: "") > 0) continue
             dao.insert(
                 MessageEntity(
                     id = dto.id,

android/app/src/main/java/com/tiangong/aiagent/ui/chat/ChatScreen.kt

@@ -74,10 +74,14 @@ fun ChatScreen(
         onDispose { lifecycleOwner.lifecycle.removeObserver(observer) }
     }
 
+    // Guard: prevent loadMoreHistory from firing before initial auto-scroll completes
+    var initialScrollDone by remember { mutableStateOf(false) }
+
     // Auto-scroll when new messages arrive (only when not loading more history)
     LaunchedEffect(uiState.messages.size) {
         if (uiState.messages.isNotEmpty() && !uiState.isLoadingMore) {
             listState.animateScrollToItem(uiState.messages.size - 1)
+            initialScrollDone = true
         }
     }
 
@@ -90,7 +94,7 @@ fun ChatScreen(
     }
 
     LaunchedEffect(isAtTop) {
-        if (isAtTop && uiState.hasMoreMessages && !uiState.isLoadingMore) {
+        if (isAtTop && uiState.hasMoreMessages && !uiState.isLoadingMore && initialScrollDone) {
             viewModel.loadMoreHistory()
         }
     }

android/app/src/main/java/com/tiangong/aiagent/ui/chat/ChatViewModel.kt

@@ -183,7 +183,7 @@ class ChatViewModel @Inject constructor(
             chatRepository.getMessages(sessionId).collect { messages ->
                 if (messages.isNotEmpty() && _uiState.value.currentAgent?.id == currentAgentId) {
                     _uiState.value = _uiState.value.copy(
-                        messages = messages.map { it.toUiMessage() },
+                        messages = messages.map { it.toUiMessage() }.distinctBy { it.id }.distinctBy { it.id },
                         sessionId = sessionId
                     )
                 }
@@ -245,7 +245,7 @@ class ChatViewModel @Inject constructor(
                 chatRepository.getMessages(latestConversation.sessionId).collect { messages ->
                     if (messages.isNotEmpty() && _uiState.value.currentAgent?.id == agentId) {
                         _uiState.value = _uiState.value.copy(
-                            messages = messages.map { it.toUiMessage() },
+                            messages = messages.map { it.toUiMessage() }.distinctBy { it.id },
                             sessionId = latestConversation.sessionId
                         )
                     }
@@ -262,7 +262,7 @@ class ChatViewModel @Inject constructor(
                     if (msgResult.isSuccess) {
                         val (msgs, hasMore) = msgResult.getOrThrow()
                         _uiState.value = _uiState.value.copy(
-                            messages = msgs.map { it.toUiMessage() },
+                            messages = msgs.map { it.toUiMessage() }.distinctBy { it.id },
                             sessionId = latestSession.sessionId,
                             hasMoreMessages = hasMore
                         )
@@ -300,7 +300,7 @@ class ChatViewModel @Inject constructor(
             val localMsgs = chatRepository.getMessages(sessionId).first()
             if (localMsgs.isNotEmpty()) {
                 _uiState.value = _uiState.value.copy(
-                    messages = localMsgs.map { it.toUiMessage() },
+                    messages = localMsgs.map { it.toUiMessage() }.distinctBy { it.id },
                     sessionId = sessionId,
                     streamingContent = "",
                     isLoading = false,
@@ -314,7 +314,7 @@ class ChatViewModel @Inject constructor(
                     chatRepository.getMessages(sessionId).collect { messages ->
                         if (messages.isNotEmpty()) {
                             _uiState.value = _uiState.value.copy(
-                                messages = messages.map { it.toUiMessage() }
+                                messages = messages.map { it.toUiMessage() }.distinctBy { it.id }
                             )
                         }
                     }
@@ -328,7 +328,7 @@ class ChatViewModel @Inject constructor(
                 if (result.isSuccess) {
                     val (msgs, hasMore) = result.getOrThrow()
                     _uiState.value = _uiState.value.copy(
-                        messages = msgs.map { it.toUiMessage() },
+                        messages = msgs.map { it.toUiMessage() }.distinctBy { it.id },
                         sessionId = sessionId,
                         streamingContent = "",
                         isLoading = false,
@@ -369,8 +369,11 @@ class ChatViewModel @Inject constructor(
                 val existingIds = current.map { it.id }.toSet()
                 val newMsgs = olderMessages.filter { it.id !in existingIds }.map { it.toUiMessage() }
                 current.addAll(0, newMsgs)
+                // Sort by createdAt then id for deterministic order (same-second messages)
+                current.sortWith(compareBy<UiMessage> { it.createdAt }.thenBy { it.id })
+                val deduped = current.distinctBy { it.id }
                 _uiState.value = _uiState.value.copy(
-                    messages = current,
+                    messages = deduped,
                     hasMoreMessages = hasMore,
                     isLoadingMore = false
                 )
@@ -387,8 +390,11 @@ class ChatViewModel @Inject constructor(
                     val existingIds = current.map { it.id }.toSet()
                     val newMsgs = roomMsgs.filter { it.id !in existingIds }.map { it.toUiMessage() }
                     current.addAll(0, newMsgs)
+                    // Sort by createdAt then id for deterministic order
+                    current.sortWith(compareBy<UiMessage> { it.createdAt }.thenBy { it.id })
+                    val deduped = current.distinctBy { it.id }
                     _uiState.value = _uiState.value.copy(
-                        messages = current,
+                        messages = deduped,
                         hasMoreMessages = roomMsgs.size >= 50,
                         isLoadingMore = false
                     )

android/app/src/main/java/com/tiangong/aiagent/ui/chat/components/MessageBubble.kt

@@ -12,6 +12,9 @@ import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import com.tiangong.aiagent.domain.model.Message
 import com.tiangong.aiagent.util.MarkdownRenderer
+import java.text.SimpleDateFormat
+import java.util.Date
+import java.util.Locale
 
 @Composable
 fun MessageBubble(message: Message) {
@@ -79,5 +82,30 @@ fun MessageBubble(message: Message) {
                 )
             }
         }
+
+        // Timestamp
+        if (message.createdAt > 0) {
+            Spacer(modifier = Modifier.height(2.dp))
+            Text(
+                text = formatTimestamp(message.createdAt),
+                style = MaterialTheme.typography.labelSmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant.copy(alpha = 0.6f),
+                modifier = Modifier.padding(horizontal = 4.dp)
+            )
+        }
+    }
+}
+
+private fun formatTimestamp(epochMillis: Long): String {
+    val now = System.currentTimeMillis()
+    val diff = now - epochMillis
+    return when {
+        diff < 60_000 -> "刚刚"
+        diff < 3600_000 -> "${diff / 60_000} 分钟前"
+        diff < 86400_000 -> "${diff / 3600_000} 小时前"
+        else -> {
+            val fmt = SimpleDateFormat("MM-dd HH:mm", Locale.getDefault())
+            fmt.format(Date(epochMillis))
+        }
     }
 }