Watching a disturbing film a second time has never been an issue with me, since the fact that they're disturbing is why I've watched them in the first place. I've probably seen Bad Lieutenant and Requiem for a Dream 4 or 5 times. I will admit that I've avoided United Flight 93 since its release because I just didn't feel like getting myself all pissed off. I have now added it to my queue.
The list contains some Oscar recognized films, Boys Don't Cry, Leaving Las Vegas and Million Dollar Baby that will have probably been seen by most of you, but if you want a challenge for this weekend I would recommend Irreversible. Read the mini review in the article then see it if you dare. You may not like it but you won't soon forget it either.