I would also say that you are not the exact same person the second time you apply. You've had 6 months to grow as a person, learn more, and think more. Also, to apply twice there must be some level of commitment to the startup ethos. You aren't just applying on a whim -- a startup is something you genuinely want to do.

At least, this is what I hope Paul Graham thinks, as I'm applying for the second time myself. ::grin::