I've always seen the "Who is back?" question with the singular verb "are", even when the question is rhetorical and the answer is clearly a plural noun but I recently came up against "Who are back?". My first thought was that it's an error but maybe I'm wrong. Can it be considered grammatically correct when the "implied subject" is undoubtedly plural?
Edit: The question I encountered is actually "Guess who are back?". Is it correct if it clearly refers to a plural subject? Besides the grammatical correctness I'm also interested in how native speakers would ask the question (in case of a plural subject).