This is a well known "feature" of HN - it will detect exact URL duplicates provided they're recent enough, probably provided they're in cache. The same story is often submitted over and over again with no way of ensuring that discussions don't get split across multiple submissions. If you follow the "newest" page you see lots of them, although usually one "wins" and the others fall off the page without an up-vote.

A recent example of the same story submitted multiple times in quick succession is the $50,000 twitter handle being lost. An example of the same story submitted many times over the years is on the front page now: https://news.ycombinator.com/item?id=7149634

Until recently I've tried to cross-reference submissions, but largely now I've given up. This is a rare lapse.

And welcome to HN.