It does, although the author probably didn't intend it. If a submission is a duplicate, you're redirected to the first submission and it gets your automatic upvote. So the first person clicks it and submits it, and then the following clickers think they're submitting it but are really voting it up. I think it'd be cooler if looked more like the Digg button (which shows the number of diggs), but you'd have to scrape for the current number of upvotes.