I was curious as to why this happens, and found an answer at poultryhelp.com:
DOUBLE YOLK EGGS: Double Yolkers appear when ovulation occurs too rapidly, or when one yolk somehow gets "lost" and is joined by the next yolk. Double yolkers may be by a pullet whose productive cycle is not yet well synchronized. They're occasionally laid by a heavy-breed hen, often as an inherited trait.You can see the cracked shell from the double-yolked egg in my picture above compared to a regular egg. The double-yolked egg was much larger--poor hen!