When using callbacks, there are times when callbacks can be added redundantly: for instance sometimes you might need to create an llm with specific callbacks, but then also create and agent that uses a chain that has those callbacks already set. This means that "callbacks" might get passed down again to the llm at predict() time, resulting in duplicate calls to the on_llm_start callback.

For the sake of simplicity, I made it so that langchain never adds an exact handler/callbacks object in add_handler, thus avoiding the duplicate handler issue.

Tagging @hwchase17 for callback review