I pre-ordered a copy of Neal Stephenson's Anathem in May, when I found it during a search for a purchase pointer to another of his books. Completely forgetting that event, I pre-ordered another copy in late June, when it got another plug and I decided that I'd likely forget to order in early September with the start of school and hockey conspiring to make me even less clueful. You'd think that Amazon would have recognized I already had one pre-order in the queue, and warned me about buying another? Oh yeah, I also told someone I wanted it for my birthday, so the hat trick should be completed with today's mail.
iTunes knows enough to warn you if you're re-purchasing something; I'm surprised Amazon can't do a match of current cart contents versus history, or at least sending an order update (asking if you want to cancel) for those of us who indulge in excessive meta-reading -- reading about what I'll be reading next.