From an audience perspective, if you go to see the same performer more than once at the same festival then it is because you like them and their material. And if you like the performer and their material then you won't mind hearing some things more than once. That said, I would not be happy to hear exactly the same set again, complete with scripted jokes and ad-libs. I suppose what I am saying is that as long as I got to hear some new stuff each set then I would be happy.
From a performing perspective, the easy option is to stick to the material you know well because you present it better. The more lesser used material put in, the more chance there is of a cock-up. However, easy does not always equal good!
I think that a mixture of previously performed good stuff and a reasonable chunk of not-yet-performed is best for both audience and performer.....