Tangential note, but it is quite interesting to see the page view counter on the Quora post increase by the second, gives you a feel of the HN audience size...
Idea for the merge mechanism.
If a user thinks a link has been submitted before; the user posts a cross reference comment whose syntax must be simple as in:
MERGE: news.ycombinator.com/link/to/article
If the merge comment gets enough upvote, then a detection algorithm automatically merges it to the other one.
I left out a couple of details, but this is the simplest way I can think of to automate the merging without reverting to clustering algorithms.
I guess this is equivalent to voting the story down the front page, but it allows for salvaging the discussion in the down voted link.