Agree that duplicate posts should cost more. Not because they're undesirable though, just because they should come with a higher burden of proof.

I purposely posted a duplicate the other day, simply because the OP chose a poor title. My post should have definitely cost more to create, but the fact that I earned more from my post (with a better title) should be a signal that my duplicate wasn't necessarily undesirable... one could even argue that the first post was the one that was undesirable.