sqlalchemy.exc.InvalidRequestError: Table 'langchain_pg_collection' is already defined for this MetaData instance. 
Labels: name: area: vector store, name: auto:bug

### Issue you'd like to raise.

what is this issue, and how can i resolve it: 

```python
    os.environ["AZURE_OPENAI_API_KEY"] = AZURE_OPENAI_API_KEY
    os.environ["AZURE_OPENAI_ENDPOINT"] = AZURE_OPENAI_ENDPOINT
    os.environ["OPENAI_API_TYPE"] = "azure"
    os.environ["OPENAI_API_VERSION"] = "2023-05-15"
    os.environ["OPENAI_API_KEY"] = OPENAI_API_KEY

    embedding = OpenAIEmbeddings()

    COLLECTION_NAME = "network_team_documents"
    CONNECTION_STRING = PGVector.connection_string_from_db_params(
        driver=os.environ.get(DB_DRIVER, DB_DRIVER),
        host=os.environ.get(DB_HOST, DB_HOST),
        port=int(os.environ.get(DB_PORT, DB_PORT)),
        database=os.environ.get(DB_DB, DB_DB),
        user=os.environ.get(DB_USER, DB_USER),
        password=os.environ.get(DB_PASS, DB_PASS),
    )

    store = PGVector(
        collection_name=COLLECTION_NAME,
        connection_string=CONNECTION_STRING,
        embedding_function=embedding,
        extend_existing=True,
    )

    gpt4 = AzureChatOpenAI(
        azure_deployment="GPT4",
        openai_api_version="2023-05-15",
    )
    retriever = store.as_retriever(search_type="similarity", search_kwargs={"k": 10})

    qa_chain = RetrievalQA.from_chain_type(llm=gpt4,
                                           chain_type="stuff",
                                           retriever=retriever,
                                           return_source_documents=True)
    return qa_chain
```

```python
Traceback (most recent call last):
  File "/opt/network_tool/chatbot/views.py", line 21, in chat
    chat_object = create_session()
  File "/opt/network_tool/chatbot/chatbot_functions.py", line 95, in create_session
    store = PGVector(
  File "/opt/klevernet_venv/lib/python3.10/site-packages/langchain_community/vectorstores/pgvector.py", line 199, in __init__
    self.__post_init__()
  File "/opt/klevernet_venv/lib/python3.10/site-packages/langchain_community/vectorstores/pgvector.py", line 207, in __post_init__
    EmbeddingStore, CollectionStore = _get_embedding_collection_store()
  File "/opt/klevernet_venv/lib/python3.10/site-packages/langchain_community/vectorstores/pgvector.py", line 66, in _get_embedding_collection_store
    class CollectionStore(BaseModel):
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/orm/decl_api.py", line 195, in __init__
    _as_declarative(reg, cls, dict_)
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/orm/decl_base.py", line 247, in _as_declarative
    return _MapperConfig.setup_mapping(registry, cls, dict_, None, {})
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/orm/decl_base.py", line 328, in setup_mapping
    return _ClassScanMapperConfig(
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/orm/decl_base.py", line 578, in __init__
    self._setup_table(table)
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/orm/decl_base.py", line 1729, in _setup_table
    table_cls(
  File "", line 2, in __new__
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/util/deprecations.py", line 281, in warned
    return fn(*args, **kwargs)  # type: ignore[no-any-return]
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/sql/schema.py", line 436, in __new__
    return cls._new(*args, **kw)
  File "/opt/klevernet_venv/lib/python3.10/site-packages/sqlalchemy/sql/schema.py", line 468, in _new
    raise exc.InvalidRequestError(
sqlalchemy.exc.InvalidRequestError: Table 'langchain_pg_collection' is already defined for this MetaData instance.  Specify 'extend_existing=True' to redefine options and columns on an existing Table object.
```

### Suggestion:

_No response_