Race condition that makes it possible to add duplicated labels
### Summary It is possible to add duplicated labels when GitLab is slow. I'm not sure how to reproduce it. It is a minor bug, but I'm creating an issue to keep track on it :smile_cat: ![duplicate_labels](/uploads/2e9701c3201e148f22e894e641e0f349/duplicate_labels.png) See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12785 /cc @smcgivern
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