When two different people open the same PR and merge it at about the same time, the first merge will work without problems. The second one will report Your task failed: Nothing to do, changes were already merged but still add a comment about the PR being merged by the second person.

I would expect when I get an alert about failed merge that there will not be the comment, and an e-mail should not be sent about it.

Example:
https://pagure.io/rpkg/pull-request/254