If I post a link in a comment for question A that links to question B, a great feature happens, question B links back to question A.
However, if I indicate an even closer relationship by flagging Question A as a duplicate of Question B, no backlink is set up on question B:
(see here, for an example Question A: What is a good free source control solution and here for the unlinked Question B: What is the best source control product for Visual Studio development? ).
Now, there may be concerns relating to search engines following links to duplicates and that hightening visibility of the duplicates via search, so they should probably be
rel=nofollow links, but it'd be much easier to decrease duplication if flagging as duplicate created a widening visibility of potential duplicates.