Streams should not be able to add duplicate contexts to state

Summary

There are some situations in which a developer might (accidentally) create/update the stream state with a duplicate partition context. This results in the stream failing on the next invocation when the duplicate is detected: https://gitlab.com/meltano/sdk/-/blob/72e7e2be7271d37c9d2c40ae0d0be3785c243876/singer_sdk/helpers/_state.py#L83-87.

Steps to reproduce

NA

What is the current bug behavior?

Duplicate partition contexts are written to a stream state.

What is the expected correct behavior?

Taps should not be allowed to write duplicate contexts.

Relevant logs and/or screenshots

NA

Possible fixes

One safeguard we could add to prevent this is to check for duplicates before they're added to the tap state in https://gitlab.com/meltano/sdk/-/blob/72e7e2be7271d37c9d2c40ae0d0be3785c243876/singer_sdk/helpers/_state.py#L95-101.