fix: load up to existing message ID when continuing a truncated response

When the user clicks "Continue" on a stopped/truncated assistant message,
the frontend reuses the existing message's ID as message_id while
parent_message_id still points to the user message.

Previously load_messages_from_db used parent_message_id, so the partial
assistant message was excluded from the DB load. The model received context
ending at the user message, generated a complete fresh response, which then
got appended to the existing partial content → duplication.

Fix: detect the "continue" case by checking whether message_id already
exists in the DB. If so, load messages up to and including that message so
the model sees the partial and generates only the suffix.

Author: Algorithm5838

backend/open_webui/utils/middleware.py

@@ -2101,7 +2101,16 @@ async def process_chat_payload(request, form_data, user, metadata, model):
     parent_message_id = metadata.get('parent_message_id')
 
     if chat_id and parent_message_id and not chat_id.startswith('local:'):
-        db_messages = load_messages_from_db(chat_id, parent_message_id)
+        # For "continue": message_id already exists in DB (the truncated response being
+        # continued).  Load up to and including it so the model sees the partial and
+        # generates a suffix rather than a duplicate fresh response.
+        msg_id = metadata.get('message_id')
+        load_up_to_id = (
+            msg_id
+            if msg_id and Chats.get_message_by_id_and_message_id(chat_id, msg_id)
+            else parent_message_id
+        )
+        db_messages = load_messages_from_db(chat_id, load_up_to_id)
         if db_messages:
             system_message = get_system_message(form_data.get('messages', []))
             form_data['messages'] = [system_message, *db_messages] if system_message else db_messages