Repeated tags

A tagging tool lets editors attach genre tags to tracks, and the same tag can be applied many times across a library. Given the full list of applied tags (case-sensitive strings), return an alphabetically sorted list of the distinct tags that were applied more than once. Tags applied exactly once are excluded. For example, ["rock", "pop", "rock", "jazz", "pop"] yields ["pop", "rock"]. Return an empty list when nothing repeats.