fix: pubsub: do not treat ErrExistingNonce as Reject

[arajasek]

Related Issues

This might help with #10906.

The ErrExistingNonce error is thrown when we already have the exact message we're trying to add in our pending mset. It is an error to try and add this message, but it is not wrong for us to receive a message multiple times, and try to add it. We should definitely not penalize peers who inform us of messages we've already added.

What's not clear to me is whether something earlier in the pipeline should be realizing that we've already processed this message. Regardless, however, I think we should be treating this case as an Ignore, not a Reject.

Proposed Changes

Lower the pubsub handling of this particular failure to Ignore.

Additional Info

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[Stebalien]

fallthrough?