A similar thing happens here if you post an identical link. Your submission is disallowed and acts as an up-vote for the original.
But too many URLs are different, sometimes subtly, sometimes not so. I've seen submissions with just an extra hash on the end. There are the submissions with all the feedburner crap cluttering it up, and so on. This was an attempt to be more thorough about detecting duplicates, doing "more properly" a job that's already done, and therefore presumably desirable.
Ah! That's good, didn't realise. The upvote is a good idea, but yeah, there's a million different ways to submit the same online content.
The function is definitely desirable, but it seems better suited as a browser plugin or such. Although it's not literally, botting it and having it contribute almost makes it mandatory for all users, versus being an optional component.
It's the root of the use case here - the functionality is great, but not all users find it desirable.
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.
If you post a link, and it exists, it simply flags: here's all the posts that already have this link. Do you still want to proceed?
Gives you the choice of jumping into the existing discussions or re-posting (eg, if the old subs are years out of date)