Double-yolked eggs are the result of a defect in the egg generation process in the hen. They can happen in any breed of hen. It is a result of two ova being generated at the same time, and then being made in a single shell.
Laying chickens require a completely balanced diet to sustain maximum egg production over time. Inadequate nutrition can cause hens to stop laying. Inadequate levels of energy, protein or calcium can cause a drop in egg production. This is why it is so important to supply laying hens with a constant supply of nutritionally balanced layer food. Feeding whole grains, scratch feeds and table scraps will cause the birds diet to become unbalanced.
After 18 days of the hatchery process the egg shell gets worn off and starts getting cracked from inside. When the chick is fully developed it breaks the inner membrane and forces pressure against the calcium shell, it breaks and out comes the chick.