We treat a submission as a duplicate if the story has had significant attention in the last year or so. This is in the FAQ: https://news.ycombinator.com/newsfaq.html.

If a story hasn't*had significant attention in the last year or so, then we don't treat it as a dupe, because it's important for good articles to get multiple chances at getting attention. Otherwise the randomness of what gets noticed on /newest would be even more dominant than it already is.