Why not something that pops up when you submit that says "Looks like your post could be a copy of these other stories. If yours is different, please check this

Why not something that pops up when you submit that says "Looks like your post could be a copy of these other stories. If yours is different, please check this box." And if you don't check the box, and your story is a copy, the system kills the link and hands all the karma to the original story.