The current implementation for adding and removing labels in GitLab gets all current labels from the MR, modifies the returned set and then sets all labels. That was likely the only way of doing it when the code was written. Looking at the Gitlab API today, there is "add_labels" and "remove_labels" on a merge_request, and I have manually verified that they work. We should change our implementation to use these instead to avoid potential races between bots.