Real v Fake

"If only I was an audience member, then I could see who's the fake one..."

When two (or more) identical entities (often people) are mixed up, it's up to someone else to choose who's the real deal and who's only faking. If there are two people — one is the real person and the other is their Evil Twin — then they will both try to convince the one who has to make the decision that they're real. The most common way to tell the difference between the two is by asking them questions that only the real one would know. If the two copies fight each other, an issue might be that they are evenly matched and have attacks or other techniques that cancel each other out, therefore negating the purpose of battling.