There's an age factor. If you submit a duplicate of something from, say, 2 days ago, your submission just turns into a vote for the other submission. But past some threshold (I don't know what it is, and I don't know that anyone other than pg does either), the system allows duplicates. I assume that's by design, as some stories are worth repeat discussions. Indeed, there are a handful of links that have been posted to, and discussed on, HN a half dozen times or more.
You can, AFAIK, also "cheat" the dupe detector by adding query string params, and trickery of that nature. I'm guessing substituting the http URL for the https URL (or vice versa) would also work, if they both exist, etc.