i like hn's method of detecting duplicate urls, in that it will allow a reposted URL after some months. content at old URLs can change, discussion can be different given recent information, and new users can see things they may have missed before.
it would be nice to automatically see links to previous discussions for a link before submitting (for the submitter) and after submitting (for users), but there are probably more important features in news.arc to (never) get implemented.
Edit: submissions have identical URLs. Is there something wrong with HN's de-duping system?