Zombie dockers

I have noticed recently that a high proportion of the spam that makes it through Spamassassin is duplicated. Between a quarter and a third on any given day will be exact copies of the same message. I imagine this is a sign that the spam networks now have more zombies than they know what to do with, so that any particular message is likely to be sent out multiple times as a horde of zombies stumble to obey, like dockers queuing for casual work in the morning.