Have you ever found a double yolked egg in a commercial box of eggs? I would be a bit surprised if you had, as they are not very common. However, there are a few things that would increase your odds of finding a double yolker, one is the age of the chicken laying the eggs. Young chickens are more likely to lay a double yolk egg than older chickens. Very large eggs laid by young chickens are also more likely to be double yolkers than standard size eggs. So, when one of my young chickens laid a very large egg, knew what I had on my hands - a double yolker.
In the above picture the two blue/green eggs were laid by my young chickens, and the brown egg was laid by an older chicken. The brown egg is about the size of a commercial large egg, the smaller blue/green is about the size of a medium commercial egg. The large blue/green egg in the upper left would not fit in a standard egg carton.
This is what I found when I cracked the egg - two yolks as predicted. Probability in action.
If you would like to learn more about the statistically probabilities associated with double yolked eggs, check out "More or Less" from BBC radio 4. It really is interesting...really!