Merging a MR that "Closes #issue" in MR description and commit closes issue twice

Summary

Merging a merge request that has Closes #123 in both - merge request description and one of the commit messages, closes issue twice.

What is the current bug behavior?

Only the first close event is effective.

What is the expected correct behavior?

Issue should be closed by merger first (to be consistent with what happens when there is no Closes #123 in commit messages), the second event should be ignored.

I created a separate issue to keep track on decision which user will close the issue, because this is not a case in gitlab-ce#21493.

/cc @smcgivern