Alternate dimensions? Plenty. I've got a dimensional doorway in my basement and frequently collaborate with alternate universe versions of myself.

As for time loops, I've never personally experienced one. I've read more than a bit about them. And we have evidence indicating one of Venkman's dimensional doppelgangers may have repeated the date of February 2nd, 1993 for an extremely prolonged period. We haven't had much of a chance to follow up on that, though.