Resolving discussions multiple times could lead to multiple system notes being created

When all discussions of a merge request are resolved by a first user, but the page hasn't updated for another user the second user can still resolve the discussions that appear open to them.

This happened in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22866/

Steps to reproduce

1. User A: Create some discussions on a merge request, leave the page open
2. User B: Resolve all the discussions
3. User A: Resolve the discussions
4. Multiple system notes are created