If you're relying on the honor system at this point (which may work for quite a while) exponential costs are probably a bit heavy handed when applied everywhere.  My thought was mainly to direct people to existing posts about the same link in case they didnt already know it was discussed before. They may still chose to post it if it's been a while since it was last mentioned (like weeks or months)

Brunswick

I like the idea of suggesting an alternative behavior. 

We are definitely going to need to do exponential posting costs in certain contexts.

k00b

Daily discussion thread

saloon

How about when you try to create a post of a link that's been posted before, it asks you if you want to add to the discussion about that same link before, or if you want to start a new discussion?  and if it's been posted before, to repost it will cost exponentially more than the last time the same link was posted.