The usual problem is that the duplicates are linked "bottom up" instead of "top down". Sometimes I suggest that the penance for missing the original idea is that the duplicator update the original with a link to the duplicate. After about one day when the duplicate rolls off the front page there is limited value in linking the duplicate to the original. On the other hand, there is some value in linking the original to its demon spawn so that NI can see that a given idea has 50 Kudos + 5 duplicates and weight accordingly.
Of course the search is a bit of a pain, most times I spot duplicates by remembering the original. Improving the search could really help, one time I had an idea declared a duplicate of an idea suggested months later. I think Google has technology to prevent this sort of thing from happening...
