reviewer is assigned twice to merge proposal

What happened:
==============
When I reassigned the reviewer of a merge proposal [1] from "Canonical Server Reporter" to "Ubuntu Sponsors" (that was already assigned) it leaves the merge proposal with "Ubuntu Sponsors" assigned twice.

I did expect it to "merge" or at leased an error message that you can not assign a person/group twice.

How/Why did it happen:
======================
1. I created a merge proposal [1] with `git ubuntu submit --target-branch debian/sid`.
 => This added "Canonical Server Reporter" as a reviewer
2. "Canonical Server Reporter" is the wrong reviewer as the merge proposal is for a foundations package
 => I added "Ubuntu Sponsors"
3. Now I wanted to remove "Canonical Server Reporter". (I was told that this is only possible through the API)
4. I thought that reassigning the "Canonical Server Reporter" to "Ubuntu Sponsors" would result in a "merged" state or at leased an error message that you can not assign a person/group twice.

[1] https://code.launchpad.net/~dviererbe/ubuntu/+source/dash/+git/dash/+merge/446503