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.
