Google translator agrees with duolingo. http://www.spanishdict.com/translate/m%25C3%25A1s also list "else" as a definition of "más".

someone else means another person, not someone more. i mean. if today i talk to you, and tomorrow i´ll talk to your friend, it´s not necessarily to talk to you before on that same day. so i say that it´s more correct to say "hablé con otra persona"

I also put "hable con otra persona". I agree that "alguien mas" means more like "I spoke to yet another person" rather than "I spoke to a different person", as I understood from the question. (Did you speak to the doctor? No, I talked to someone else.)

Hmmm........le hablé a otra persona?

This is what I had - Yo hablé a alguien más - and the Yo was crossed out - any idea why since the correct answer was Yo hablé con alguien más