This is in the FAQ: we don't count submissions as dupes if the story hasn't had significant attention yet. In other words, reposts are ok until the article has had a significant discussion, or at least significant upvotes. This is to give good articles more than one chance at getting attention, since it can be pretty random which articles get noticed on /newest.

In this case, we actually boosted the submission into the second-chance pool (see https://news.ycombinator.com/item?id=11662380), which lobbed it on the front page. The reason we picked this post rather than yours is that it was the first submission of the article. (It doesn't look like it right now, because the timestamp was adjusted to be the resubmission time, but you can tell from the ID in the URL.) In the future we want to do some sort of karma sharing so multiple submitters can get credit, but that's not implemented yet.