The main advantage I see to not worrying about duplicate threads is that it encourages those not only new to the board, but those new to jazz. The boards I go to that seem to be growing, living boards don't worry about duplicate threads, and newbies seem to stick around a lot more, growing in knowledge and in contributing to the board. The boards that worry about duplicate threads and insist that newbies use the search feature, post on the appropriate thread, etc., seem to run of the noobs quickly, and soon become an "old gang" board. I agree with Tom Storer in that, in this day of facebook, bulletin boards may be up against it, but there are boards that are doing fine.
Of course, the other advantage to not worrying about duplicate threads is less work for you...
As I see it duplicate threads are a nuisance when they're posted shortly after the original thread, especially when there's a discussion going on in the earlier one. The starting point here is to keep all info and opinions on a particular subject in one thread as much as possible, which makes a discussion a lot easier to follow (and take part in) than when it is fragmented between threads and it also makes it easier to do a search.