I went to college with a guy who introduced himself under two different names to different groups of people. I don't know why.
On several occasions the groups converged, which led to lots of conversations along the lines of "You really look like a guy I know called John" or "Are you sure you're not related to Steve? You must know him. He's studying English Literature..."
As far as I know, he never 'fessed up, and was never caught.