Thanks for all the responses. I don't know much about reloading, but I know he uses one of the expensive reloaders that works like an assembly line, and has several hoppers holding all of your bulk supplies.
He noticed the double powder right after it happened, but the loader was dropping all of the shells into a bucket below. I'm not sure why he just didn't rake off the top layer of shells.
I think I'll take all of your recommendations and shoot some Remington ShurShots.