I vote for do nothing. If the story is exactly the same just from a different source, it should be downvoted. Some duplicate stories have a slightly different slant and thus encourage a discussion of the subject from a different perspective. The most obvious example is the AirBnB story from the financial times. There is a lot of overlap in the comments from this submission and previous ones, but there are also different perspectives. I think for stories that are this big its better just to leave it alone instead of attempting to cull the duplicate submissions.
I had forgotten that you can't downvote submissions (after 2.5 years I still haven't crossed the downvote enabled threshold). That does change my opinion somewhat. I do remember an alarmingly large number of duplicate G+ stories during its launch.
Given that the main way to manage duplicates (by downvotes) is disabled, I could support some other mechanism. I like the idea mentioned in the comments about grouping similar submissions and have the comments merged.
