With google's machine learning brain trust, I think they could still do a pretty good job deduping. Maybe not perfect, but I'd bet on them to win an arms race.

Edit: ah, codeflo and EGreg are right. I was just thinking about the task of determining that the images serve the same role in each message (which I'm sure google could do a good job of). But (as they point out) in the "Dear <user>" case they'll still have to show the right image to the right user. Although, as Nacraile and jaxn say, if they load all those images eagerly they'll remove the value of those unique tracking images, and impose a cost on the sender.