Diff comments get posted multiple times

When adding a comment to a diff, it got posted multiple times:

This happened in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2885/diffs#note_41653688 (but I have since deleted the extra copies to keep the MR workable).

I was on a bit of a wonky internet connection, so I might have clicked the "Comment" button multiple times when getting impatient.

@filipa Noticed as well in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14303/diffs#note_41582221

The last time I saw a similar problem was due to a bad Sidekiq state: https://gitlab.com/gitlab-com/infrastructure/issues/1712

I suggest we look for the NewNoteWorker logs for the note ID in question.

I see no evidence Sidekiq died and retried this job: https://log.gitlap.com/goto/1a6ee4b7bb09afd9a42d74d7c1269b05

Perhaps next time leave the duplicate notes so we can track the IDs.

The duplicate notes are still available here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14303/diffs#note_41582221 (duplicate: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14303/diffs#note_41582224)

So the note IDs are different, that's why I think the frontend made multiple posts, possibly after the user clicked 'Comment' multiple times.