I see double yolk eggs more often when I buy jumbo eggs than any other size. Usually, it's an 'exciting surprise' but you don't actually get all that much more yolk. It looks the same to me, just split in two. I liken it to the fact that twins are usually smaller in one pregnant woman than one baby would be. Consider there's only so much room in that egg shell....
If you buy jumbo eggs consistently, I bet you'd find one, sooner or later.
How can we sleep while our beds are burning???