Hair worn in the same style, though notably, very different colors (black vs dyed purple). It’s would be an odd thing, though, wouldn’t it, to cheat via “twin” and then so blatantly have the twin appear with you? Unless having the twin appear was to throw off suspicion (“if she ran part of the race, why would we show her to you”) or a red herring to divert attention from a different cheating method, by a known cheater. (“Those message board dudes are psychos, they’re even claiming my bestie ran some of the race for me just because she dressed like me! …no, don’t ask about the bike you found discarded in the woods.”)
One could probably go mad thinking of all the potential layers of bluffs and double bluffs.