chx has 8000+ karma whereas OP has ~600, so scratch that as a possible explanation. I would also guess it mainly depends on the time of day.

The question rather is, why would the system allow the same URL to be submitted twice. In the past when I tried to submit something already in, I was redirected to the existing post and my submissions was converted into an upvote.

A five-day-old HN submission is effectively dead. Would make sense if the behavior you describe only happens to dupe urls within 24hr of each other if the first one hit a certain threshold of traction. Else makes more sense to allow the dupe submission to give it another shot.

You get redirected to an existing post if it has already had significant attention or is less than 8 hours old. Otherwise reposts go through.

Why don't you just bump the post to newest if someone attempted to resubmit it?

I don't understand what you mean exactly.

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.

Thanks, I see what you mean now.

If we did that then /newest would no longer be a simple chronological log of everything posted to HN.

Indeed, there seems to optimal times for posting: http://hnnotify.leknarf.net/

Karma doesn't affect it. It's really mostly random.