I would agree that it would be nice to cross check the names/addresses on the mailing list. Since it's all sent out by bulk mail, the cost is not likely significant, but every little bit helps.
I did receive duplicate guidebooks, took the extras to the DIScon/DVC meet and gave them to those in attendance who hadn't yet received one.
Until just recently, I did not get duplicate mailings and am puzzled what change occurred with the list. I assume it will get back to minimal extras soon.