There are different ways of looking at this, of course, but HN's rules are clear: (1) please post the original (as devy quoted above), and (2) a post is a dupe if the story has had significant attention in the last year or so (https://news.ycombinator.com/newsfaq.html).

These rules maximize the overall quality of the site. It's certainly true that not everybody sees every post; but if we went with that logic, there would be no such thing as a duplicate at all. That would be bad. Front page space is the scarcest resource on HN, and needs to be distributed across as many high-quality stories as possible.

A great tool for running across interesting stories that you didn't see on HN the first time round is the 'past' link in the top bar, which lets you browse HN's top stories day by day.