Check for branch duplicates after creating changeset specs

Created by: mrnugget

This is a follow-up to #442 and ensures that changeset specs are not getting silently lost by validating that multiple changeset specs in the same repository have different branches.

I decided to make this a separate step after the execution of the steps so that users can leverage the cache. That allows them to change the campaign spec and then rerun the command after they get this error, vs. the execution being aborted after running into this error (if we'd do the check inside executor).

Screenshot: