When merging don't re-resolve issues that were reopened

If you have a commit on a branch that resolves an issue using the "fixes #xxx" format in the commit message, the issue gets correctly resolved. But if someone reopens the issue afterwards and someone else merges the branch into another one (e.g. by pull request) the Issue gets fixed again, by the same commit, even though nothing changed between reopening and resolving.

I looked if there exists another issue like this one, didn't find anything.

Easy steps to reproduce:

1. Create branch "branch-x"
2. Create issue xxx
3. Create commit on branch-x with commit message "fixes #xxx"
4. Create pull request for branch-x to master
5. Reopen issue xxx
6. Merge pull request.

Result: issue xxx is resolved.

Expected result: A single commit can't resolve the issue twice.