fix: deduplicate re-delivered session sends on the client message id

Typed-3-shown-5 forensics (project 000): the WS client queues session_send
payloads while disconnected and flushes the queue after a reconnect, and the
server minted a fresh row id per delivery — so one typed message could land
as several user turns, each dispatched to the engine.

Every send now carries a client-generated ui_message_id (dispatchSessionSend
injects one when the caller didn't). The handler persists the user row under
that id and answers any later delivery in the same channel with an idempotent
ack instead of inserting and dispatching again. Because the row id now equals
the optimistic bubble's ui_message_id, the echo also merges with the local
message even after the transcript sync rewrites row metadata.

Same text intentionally sent again gets a fresh id and still starts a new turn.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_session_integration.py

@@ -791,6 +791,67 @@ class TestWSHandlerSessionSend(unittest.IsolatedAsyncioTestCase):
         metadata = json.loads(rows[0][2])
         self.assertEqual(metadata.get("ui_message_id"), "ui-message-1")
 
+    async def test_session_send_duplicate_delivery_is_deduplicated(self) -> None:
+        """A re-delivered send with the same client ui_message_id (WS pending-queue
+        flush after a reconnect) must not create a second row or a second turn."""
+        ws = MagicMock()
+        dispatched: list = []
+
+        def _record_dispatch(_task_id: str, coro: Any, **_kwargs: Any) -> None:
+            dispatched.append(_task_id)
+            coro.close()
+
+        self.handler._track_session = MagicMock(side_effect=_record_dispatch)
+
+        payload = {
+            "project_id": "test-project",
+            "task_id": self.task_id,
+            "content": "你的交付文件在哪里？",
+            "metadata": {"ui_message_id": "ui-dup-1"},
+        }
+        await self.handler._handle_session_send(ws, dict(payload))
+        await self.handler._handle_session_send(ws, dict(payload))
+
+        channel_id = f"session:{self.task_id}"
+        cursor = await self.chat_store._db.execute(
+            "SELECT message_id FROM messages WHERE channel_id = ? AND sender = 'user'",
+            (channel_id,),
+        )
+        rows = await cursor.fetchall()
+        self.assertEqual(len(rows), 1)
+        # The row is persisted under the client id so later copies are detectable.
+        self.assertEqual(rows[0][0], "ui-dup-1")
+        self.assertEqual(len(dispatched), 1)
+
+        dedup_acks = [
+            call.kwargs
+            for call in self.handler._send_ack.await_args_list
+            if call.kwargs.get("deduplicated")
+        ]
+        self.assertEqual(len(dedup_acks), 1)
+        self.assertEqual(dedup_acks[0].get("message_id"), "ui-dup-1")
+
+    async def test_session_send_same_text_new_id_is_a_new_turn(self) -> None:
+        """Deliberately re-asking the same question (fresh ui_message_id) still works."""
+        ws = MagicMock()
+        self.handler._track_session = MagicMock(side_effect=self._discard_session_dispatch)
+
+        for ui_id in ("ui-ask-1", "ui-ask-2"):
+            await self.handler._handle_session_send(ws, {
+                "project_id": "test-project",
+                "task_id": self.task_id,
+                "content": "进度怎么样了？",
+                "metadata": {"ui_message_id": ui_id},
+            })
+
+        channel_id = f"session:{self.task_id}"
+        cursor = await self.chat_store._db.execute(
+            "SELECT message_id FROM messages WHERE channel_id = ? AND sender = 'user' ORDER BY timestamp",
+            (channel_id,),
+        )
+        rows = await cursor.fetchall()
+        self.assertEqual([row[0] for row in rows], ["ui-ask-1", "ui-ask-2"])
+
     async def test_session_send_auto_titles(self) -> None:
         """First message should auto-generate title from content."""
         ws = MagicMock()