Our band has usually gets one 30-45 minute set at most of the festivals we play at. However, we have occasionally performed two 45 minute sets during a 2 day festival. We wouldn't think of performing the same material!

At one 2 day festival I attended, I noticed a headliner group that played the identical set not just during the same weekend, but in two different years. It does help sell more recording to be able to say "This is from our (1st, 2nd, 3rd etc) CD" but I was disappointed that such good musicians did not prepare new material.