I see a lot of comedy in NYC and often hear comedians tell the same joke at different venues. You can sometimes hear a lack of enthusiasm in repeated tellings, or the comedian forgets some element of the set-up, mumbles some part they know but the audience doesn't. Personally, I hate hearing the same joke twice. That's part of why a lot of comedians on TV focus on the news and others do crowd work. Nobody wants to hear a repeat, especially yourself. The worst is when a comedian comes out who hasn't been watching the show, does some crowd work and tries to do the same jokes another comedian already did with the crowd. You've always got to be fresh and play off of what has already been said.