Peer reviews on check-in, occasional code reviews… that’s where to educate a developer by means of human communication about copying and pasting code. A mechanism like this is just asking to be gamed. If forced on someone it is the equivalent of a recurring nasty sarcastic note on your screen versus an honest constructive chat.

I understand why you might want an automated nanny like that but if there must be something like this it should be a quality gate before check-in (like fxcop / stylecop) *not* something that criticizes the work as it is being written.

Maybe some kind of fuzzy comparison between projects would help here, to spot when they first add the new copy back into the solution.

The dialog could say: “It looks like you’re trying to vandalize everything we hold dear.”