I'd never, ever, give the same set twice at one festival, but it's OK to repeat particular songs especially if they're requests or 'killers'. If you're doing as many as five sets you might find you build up the confidence to try out some of your less familiar material as you go along - that's certainly what I do by the end of a long festival like Whitby or Sidmouth. Though of course you wouldn't want to make your last set of the weekend a whole bunch of stuff you don't really know!