The AACR Abstract Submission Guidelines appear to specifically prohibit the submission of an abstract that has been already published or even already been presented at a large annual conference.

Yet, it seems to me that it is pretty common to visit multiple conferences in a year and not present completely different research each time. Is this policy simply an outmoded formality that is not being enforced? Is this exclusive to AACR conferences? Does anybody know of cases where abstracts were rejected because they were previously presented at a different conference?