Actually, there is. If you try to submit an URL that has already been posted, then you upvote the original submission. But it seems there's some sort of expiry

Actually, there is. If you try to submit an URL that has already been posted, then you upvote the original submission. But it seems there's some sort of expiry after which reposting the URL creates a new submission. Also, sometimes there's problems when 2 different URLs point to the same article.