I think a more useful metric than "number of votes since submission" would be "number of votes in the past hour/day/whatever." That way an item could be submitted days in the past, but still get on the front page if it had enough upvotes recently. For testing purposes, this could be an alternate front page like http://news.ycombinator.com/classic
Another option would be to have moderators manually de-dupe stories or fix them to link to the original source. While they're at it, the mods could ban or reduce the karma of users who submit blogspam or duplicate stories.