Caching issue is unlikely, given that HN runs as a single thread on a one server, writing to a flat-file backend. As far as I'm aware the main cache involved is the OS's page cache.
To your main point, the dupe detector seems to be fairly limited; I think it only checks for the same URL in the past 12 hours or 100 posts or something. It seems to be just to keep the same post from showing up multiple times in /new.
To your main point, the dupe detector seems to be fairly limited; I think it only checks for the same URL in the past 12 hours or 100 posts or something. It seems to be just to keep the same post from showing up multiple times in /new.