Some posts get reposted a lot see this website for example https://news.ycombinator.com/from?site=lihaoyi.com some were posted 7 times.
This thing is that almost any well written (and not too much technical) post can reach the frontpage it only requires 3-4 people who like the post enough to upvote it during the first 30 minutes. As a result reposting works for those kind of content. And as it is not forbidden, people do it