Essentially, as I understand it, if multiple people submit the same exact URL in a short period of time, they count as votes on the first submission (this may well be completely wrong, but that is how I believe it works).

After a period of time, new submissions are their own post, to allow submissions that did not get visibility have a chance at being seen. This is also sometimes done manually by the mods if they see something particularly interesting that was missed (not sure about the last point though again, I believe it to be true).

I've had the same thing happen with a few of my submissions. On the one hand: cool, the story I liked is now being seen. But OTOH: not so cool, I like the HN metagame too.