i only had one...so far. i don't know what the second one means, but i'm sure it's not a bad sign. they wouldn't invite you back if they didn't want to hear more from you! just take it as yet another opportunity to show them how super cool you are. rock on!
I don't know of any of my classmates who've had second interviews (i'm sure it's cause i haven't been bothered to ask enough of them), but it can't be a bad thing. Just out of curiousity, who was your interviewer?