When I post a link to HN that already exists, it auto-votes the prior story for me. There must be some time threshold after which it's allowed to report the same link.
It checks submissions against the links that are cached in memory -- you can resubmit the same URL if a page containing a link to it has not been requested since the server last crashed.
You can submit links that nobody's looked at recently (modulo the consistent crashiness of Arc). As a hack I think it's clever and terrible all at once.
