Remove duplicated RAG search results

[juntao]

We often have multiple vectors pointing to the same source document. When I have --qdrant-limit 3, it could find 3 different vectors that has the same source and hence include the same document in the system prompt 3 times.

The RAG server should check and remove duplicated source before inserting into the prompt. For example, if all 3 results are the same source, it should only insert the source into the system prompt ONCE.