I'm not convinced that multiple submissions is a bad thing. We're all going to continue to get out information from different sources, and the discussion that follow a particular submission may have as much to do with the source as the story. Some people might place more trust in one source over another or one poster/submitter over another, and I think that is why the multiple submissions problem hasn't already worked itself out on its own. Any sort of automated merge/filter would be a form of editorializing that could squelch valuable discourse.