1. an illegal or nasty counterpart
2. a rouge wi-fi access point that appears legitimate but is actually used to eavesdrop over the internet
If that is an evil twin, I don't want to know what the real thing is like.
A common tv formula in which an actor will play his well-known and beloved character AND that character's identical yet opposite evil twin through trick camera shots and the like. But so we don't get confused, the actor will have a goatee when playing the evil twin.
Spock's evil twin looks like a reject fratboy douche with that goatee.