My wild guess, based on other examples, is that duplicate submission detection lapses after a certain interval, so that there can be fresh discussion of interesting topics. But I haven't checked the source code to see if that is indeed how it works.

There is enough churn in membership here on HN that if the software lets a submission post as a new thread, I'm willing to treat it as a new thread. I try always to post canonical URLs when I submit links myself, the better to activate the duplicate detector if necessary. /metadiscussion

Had to go into the database for this one: http://news.ycombinator.com/item?id=58424