Not sure how familiar you are with the way Digg did their analysis, but I see that the "related questions" when submitting a question here are poor matches to say the least. Hence why some may get duplicate questions. And lets face it, people are naturally lazy, and may not hit the search function before asking a question.
The way Digg did this was after you try to submit the story, it would list the related questions, and make you confirm. This allows an additional refinement based on Tags. And yes, I realize many people will click through a confirmation as well, but it may filter.
Does StackExchange allow for such a mechanism? Could this perhaps improve the noise to signal ratio of the "related questions"?