yeah I asked this ques last year at fah forums, and the response was..

if the preferred deadline is passed, then the wu goes out again... so the work gets duplicated by another machine...
so if everyone turned in wu's bt preferred and final, then all wu's would get sent out twice..

but that's not our fault as users, that's fah's policy. we can't really do anything about it except try to meet the preferred deadline.

this actually doesn't really affect the gpu2's, but it did the smp's. because many slower dual cores barely made the preferred deadline. in fact I was folding on a P4 HT that barely made the final deadline for a while, which means every wu that machine did created duplicate work... but I got points for it anyway, since it met the final deadline.