fix: stop system prompt being duplicated

backend/utils/misc.py

@@ -82,7 +82,7 @@ def add_or_update_system_message(content: str, messages: list[dict]):
     """
 
     if messages and messages[0].get("role") == "system":
-        messages[0]["content"] += f"{content}\n{messages[0]['content']}"
+        messages[0]["content"] = f"{content}\n{messages[0]['content']}"
     else:
         # Insert at the beginning
         messages.insert(0, {"role": "system", "content": content})