This is a frequently asked question. The answer can be found in HN's FAQ: https://news.ycombinator.com/newsfaq.html. Reposts are allowed if a story hasn't had significant attention yet, because we there's a lot of randomness in what gets noticed on the /newest page and we want to give good stories multiple rolls of the dice.
Lots more explanation over the years: https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme...
Now let's resubmit http://example.com a few days later. It should be on top of https://news.ycombinator.com/newest again. But not as a new post. No, the first submission should be moved up here.
Obviously after some time / amount of comments this makes no sense, people resubmit years old links etc but thresholds can be set.
If we did that then /newest would no longer be a simple chronological log of everything posted to HN.