Notify users when a job is a duplicate #748
Description
We need to have the API tell you that you are submitting a duplicate job, or else it's easy to get stuck in loops like this one where you think you're fixing an issue and getting the same error in return, but actually you're submitting a duplicate and just seeing the last error log repeated back at you:
purescript/registry#535
We should instead reply only with an error that this is a duplicate job and a pointer to the logs for that job. In this case the user was making adjustments to the code at the tag v2.0.0 to preserve the initial release as a 2.0.0 release, but we treat tags as immutable and hence treated these as all duplicates.