Interviewing is a different skillset than most development positions ever use; you can be a good developer but a terrible interviewer. You'd be great at the job, but you get rejected in favor of someone else that put some more practice into skills that are applicable to interviewing (i.e. whiteboard programming, working without reference material, etc).

You go into the interview, having not studied. Your twin studied and ended up being more impressive during the interview. Out of the two of you, who will get the job?