1. A validation restricts a protected branch so it can only have a
   single access level for a given role or user.

2. More context:

    - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581#note_14051822
    - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581#note_14052756
    - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581#note_14056066
    - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581#note_14058676