Well, I've been browsing the internet in search of what causes double yolks and I had to share this 1940's photo of a little girl washing eggs that I stumbled across.
I love this picture! Just click on it to see the full size version.
Well, anyways, back to my original topic. Wikipedia had this to say:
Double-yolked eggs occur rarely, only leading to observed successful hatchings under human intervention, as the unborn chickens would otherwise fight each other and die. (hmmm... wonder how they "intervened" in an unhatched egg?)
Then a chicken site I stumbled upon had these interesting tidbits:
Double yolks are a "mistake" in the chicken's reproductive system that sometimes happens when a hen just starts laying eggs and her system is still trying to figure out how to do it correctly. It may also be hereditary.
Occasionally, an egg contains more than two yolks. The greatest number of yolks found in one egg is NINE. Record breaking eggs are likely to be multiple yolkers. The Guinness Book of Records lists the world's largest [chicken] egg (with a diameter of 9 inches/22.5 cm) as having five yolks and the heaviest egg (1 pound/0.45 kg) as having a double yolk and a double shell.
The site that I found the information on had some pictures of really strange eggs. Like a spoon shaped egg or an egg within an egg! You can see for yourself by going to this link.