They are not, it's even explicitly allowed to repost links that didn't see activity as long as it is not overdone. I think currently the system only rejects perfectly identical URLs for a few hours.

This doesn't always work, for whatever reason. I've had this as well, but there's not much you can do. The Upvote game isn't that important, so don't lose sleep about it. Check the FAQ first, but send an email to HN support if it's a bug.