It could be a consequence of a big newcomer community. The likelihood of this being reposted and ending up on the first page tends to 1 after a certain critical mass of HN'ers who haven't seen it posted is reached.

Something like Stack Overflow's similar title algorithm could come in handy but it's a divergence from the minimalism of the site.

It's a feature that doesn't add any complexity to the use of the site (though it does add a fair amount of complexity on the backend)

Doesn't "Hacker News" imply links that are only new? How about an "old" flag that buries the submission and bans the user from adding new submissions for a few days?

A no-posting-ban would be way too strong for this kind of error. If we're going to make the distinction that something is old, first of all, pg ought to add better duplicate detection (I've seen cases where the same link can be submitted multiple times if enough time has passed, I think). Secondly, posting something old is almost always an innocent mistake; so if we're going to criminalize it, we ought to at least present the poster with a digg-like "Are these items similar to what you posted?" thing to give them a chance to fix it.

Considering that Steve's speech doesn't really say anything original, we might as well have disallowed linking to it the first time, if old is bad.