I seem to remember there's a system in place to prevent duplicates (where the submission of a duplicate link results in the upvote of the original story), but maybe that's limited to the first 24h? Idk, HN works in mysterious ways
It's a settled enough part of HN that you'd do better emailing hn@ycombinator.com about it than you will litigating it in public. I don't care either way, I'm just explaining the behavior to you.