I usually follow the childhood adage "First time it's funny, second time it's silly, third time's a spanking." Probably better summed up as http://www.c2.com/cgi/wiki?ThreeStrikesAndYouRefactor. Problem is, after 2 repetitions you often don't know what the real abstraction behind the duplicated code is, and so you end up with convoluted objects or closures to swap out parts of behavior, while there's a more natural abstraction just around the corner. If you mercilessly refactor after only the first duplication, you might not see it after the second, because the original code has been abstracted enough that it doesn't look like the third repetition.

This also squares with a lot of IBM's experience in The Mythical Man Month, i.e. code needs at least three clients to really be reusable.