This plays into RiderOfGiraffes' experiment with leaving comments that tied together related (or identical) discussions, which I was rather sad to see him discontinue (especially in cases where two threads were discussing different aspects of a story). I don't recall, though; was he automating that, or was it a purely manual effort?
I suspect a "closed as duplicate" system, ala StackExchange, could work well here, assuming a high enough bar were to be set for who could do (or vote for) such a thing.