I like the idea of submitting "costing" karma, but maybe you get a couple freebies a day. Maybe an escalating cost schedule so it penalizes people who submit their 15th story as opposed to their 5th.
Re #2, I always thought there was a unique url filter on submissions, but I've seen a couple repeats recently.
I think there's a unique URL filter on submissions; but not a unique story filter on submissions (which would be a rather difficult AI problem).
Even with different URLs, there's really no need for 10 different stories about the MacBook Air to be posted here -- it would be much better to have one Hacker News item and have URLs to other articles posted in comments.
Re #2, I always thought there was a unique url filter on submissions, but I've seen a couple repeats recently.