"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."
There is also a section on wikipedia about Lou Mitchell's Restaurant in Chicago, Illinois using only double yolked eggs. Interesting!
So Chuck had the excitement of cooking and eating a double yolked egg today and I learned something new? Have any of you ever seen a double yolked egg before?