EnsembleRetriever with id_key accumulates scores for within-retriever duplicate documents

### Submission checklist - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [x] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description `EnsembleRetriever` with `id_key` set does not deduplicate documents within a single retriever's results before computing RRF scores. If a retriever returns multiple documents sharing the same `id_key` (e.g. multiple chunks from the same source in a vector store), each occurrence adds a separate RRF score contribution, giving longer documents a scoring advantage over shorter but equally relevant ones. ### Use Case RAG pipelines that chunk documents and store them in a vector store. When the vector store retriever returns multiple chunks from the same document, that document's RRF score gets inflated relative to documents that only matched once — not because it's more relevant, but because it has more chunks. ### Proposed Solution Add a `dedup_within_retriever: bool = False` option to `EnsembleRetriever`. When True and `id_key` is set, deduplicate each retriever's results by `id_key` before scoring, so each document contributes at most one rank signal per retriever. Default is F…