Let's presume http://example.com was never submitted. So, when it is submitted, it is on the top of https://news.ycombinator.com/newest but it doesn't get much attention.

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.