I'm pretty sure this is wrong. I think what you're talking about is HN's filter that prevents you from posting a story whose URL is identical to the URL of a story that's currently in memory. If you try to submit a URL that's already in memory, it gets converted to an upvote for the original.

That doesn't prevent you from either posting the exact same URL once the original story has been purged from memory (probably after a couple of weeks) or posting a slightly different but completely equivalent URL. For example, IIRC the following are all counted as different URLs:

• http://google.com

• http://www.google.com

• http://google.com/

• http://www.google.com/

• http://google.com?

• http://www.google.com?

• http://google.com/?

• http://www.google.com/?

I imagine https:// would add eight more variants, but I haven't ever heard of anyone trying it in real life.