Actually the link checker only checks vs. links that are currently loaded into memory.
links loaded into memory - how does that work? Can you please explain?
Anything that has been accessed within the past x amount of time and hasn't fallen out of cache yet. For more info, check http://github.com/nex3/arc/blob/master/news.arc around lines 1485, 1550, 233.
That would probably work terrifically if there was an eviction algorithm other than crashing at least once per day. (If you try to comment and get "Unknown or expired link", it's because news.arc crashed between the page load and the POST)
Not only that, but if it does find a story to the same link, it will upvote it.
I can confirm that this works if the URL is exactly identical - it happened to me and I was quite pleased.