Thanks, I thought HN detects duplicate submissions and shows that it already exists. Thanks for the heads up anyway. Will have to do a check on HNSearch before putting up things.
It's supposed to, and often does. In this case the URLs appear to be identical, so I don't know what's gone wrong. Possibly a restart, and the older item hadn't been loaded into memory. I'm pretty sure the algorithm only checks against URLs in cache.
http://news.ycombinator.com/item?id=2682320