Notify if duplicate BBM has been queued

Currently we log a warning on enqueuing a redundant BBM. We re-queue BBMs for few reasons and it's documented to make sure the existing migration is deleted before re-enqueuing. But it would be better if we can notify about this from db-testing pipeline as an MR comment.

Usecase: gitlab-org/gitlab!137261 (diffs) requeued a BBM without deleting the existing one and we found this only after getting deployed to prod.

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