If I recall correctly, this heuristic was found experimentally: Once upon a time, reposts were allowed, and so if something was topical and popular, sometimes

If I recall correctly, this heuristic was found experimentally: Once upon a time, reposts were allowed, and so if something was topical and popular, sometimes you'd see multiple items on the front page pointing to the same URL. Sometimes the second or third posting would get more karma simply because of an editorialized title. This was undesirable, so they added the repost merging. Later, an emergent behavior was that posts that report on something found on another site and had been previously posted would find their way to the front page. Some subset of HN visitors would be newer than the original post, and simply upvote something they hadn't seen before. But also there would be times where the post would be interesting with the added value of hindsight, or would provide context to the topic-of-the-day. So at some point it was decided that reposts would be allowed in the long-term, since sometimes they had value. Thus we're at the state we have today.