MongoDocumentStore reload error

The goal of the task is load multiple document store them into MongoDB and Chroma db , used them when we need in future but based in
MongoDB Demo you need vector_id to load data back but in this case how we used multiple document , what if I want to more data into existing document.

def build_automerging_index(
    documents,
    llm,
    embed_model="local:BAAI/bge-small-en-v1.5",
    chunk_sizes=None,
    
):
    print("Create Index Record")
    chunk_sizes = chunk_sizes or [2048, 512, 128]
    node_parser = HierarchicalNodeParser.from_defaults(chunk_sizes=chunk_sizes)
    nodes = node_parser.get_nodes_from_documents([documents])
    leaf_nodes = get_leaf_nodes(nodes)

    merging_context = ServiceContext.from_defaults(
        llm=llm,
        embed_model=embed_model,
    )
    # create (or load) docstore and add nodes
    db = chromadb.PersistentClient(path=DB_PATH)
    chroma_collection = db.get_or_create_collection("test")
    vector_store = ChromaVectorStore(chroma_collection=chroma_collection)

    # mongodb index store and document store
    index_store = MongoIndexStore.from_uri(
        uri=MONGO_DB_URL, db_name="audit", namespace="llm"
    )
    docstore = MongoDocumentStore.from_uri(
        uri=MONGO_DB_URL, db_name="audit", namespace="llm"
    )
    docstore.add_documents(nodes)
    storage_context = StorageContext.from_defaults(
        docstore=docstore, index_store=index_store, vector_store=vector_store
    )
    storage_context.docstore.add_documents(nodes)

    automerging_index = VectorStoreIndex(
        leaf_nodes, storage_context=storage_context, service_context=merging_context
    )
    storage_context.persist()

    # test with vectordb and without db
    # automerging_index.storage_context.persist(persist_dir=save_dir)
    vector_id = automerging_index.index_id
    print("vector_id",vector_id)
    return automerging_index