Some articles on yolked:

Egg Yolk - Double-yolk Eggs
... Some hens will rarely lay double-yolked eggs as the result of unsynchronized production cycles ... hens to have a higher propensity to lay double-yolked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay ... Usually, a double-yolked egg will be longer and thinner than an ordinary single-yolk egg ...