I'm sure others must have hit this problem after being on EL&U for a while. With some questions which crop up repeatedly, it can be quite difficult to find an earlier related or exact duplicate, even if you know it exists.
As an example, I know there are a lot of duplicate/overlapping questions covering the subtleties of conditional tenses (could/should/would, might/ought, etc.). But it's a waste of time searching for any/all of those words because just about anything returns thousands of results.
It's pointless sorting any results by "most votes" because many of the "definitive" answers are quite old, but have few votes. Often they effectively "lie dormant" because more recent new users never come across them to upvote. There may be other ways to improve the situation, but I suggest...
Increment a count against existing question when a new one is closed as "exact duplicate" thereof, and make that count available as a "sort results" key in searches.
It seems to me the questions I'm interested in should rapidly float to the top with such a system. Added to which I'd expect a significant improvement in Answer quality over time. Later contributors would have a vested interest in vying for upvotes on pages they'd expect to be visited often, well into the future. And all visitors would be likely to upvote good answers.
I'm saddened by the fact that the most upvoted Questions of all time are a somewhat anal discussion about nested [sic]'s, and a closed Dalai Lama joke. This hardly reflects what I like to think of as EL&U's laudable aim of becoming a consultable reference source for future visitors. Anything that might raise the profile of genuinely important / repeatedly relevant questions over these "viral" distractions must surely be a good thing.