Steps to reproduce

1. Ack the same message multiple times on a streaming pull subscription.

This assertion fails when ack duplication occurs:

There is no de-duplication of acks in the dispatcher, and because ack_reqs_dict is a Dict[str, AckRequest], the latest AckRequest will replace an earlier request.

The solution is either to change ack_req_dict to allow for multiple AckRequests for the same message, or to de-duplicate acks in the dispatcher.