That's a fair argument. A solution would be to increase the initial weight for same link posts every time they're posted. That would decrease the time they spend in the front page exponentially.
You could easily bypass this, of course, but I don't think most people are trying to hack the front page, they just found something interesting and they're sharing it with us.
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:
You could easily bypass this, of course, but I don't think most people are trying to hack the front page, they just found something interesting and they're sharing it with us.