While assignment of a user to a team (bracket) in CTFd should be possible only once, at the registration, a flaw in logic implementation allows an authenticated user to reset it's bracket and then pick a new one, joining another team while a competition is already ongoing. This issue impacts releases from 3.7.0 up to 3.7.4 and was addressed by pull request 2636 https://github.com/CTFd/CTFd/pull/2636  included in 3.7.5 release.