Diff comments get posted multiple times

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

Screen_Shot_2017-09-27_at_17.36.22

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