It really is obscene how many useful answers I've found on threads that were subsequently closed as duplicate -- with a link to a question that's not even close to the same.
SO hides questions with answers... but still retains them and allows logged in users with enough karma to see them?
That is insane.
I.e. there isn't a "closed for being homework" category
Consequently, duplicate served as the go-to but inappropriate "there's something wrong with this, and I don't want to deal with it" button.