Eggs that contain two yolks inside are the result of a hen's body releasing two embryos during the same general time period. If you crack open an egg and find that it has two yolks, there is no reason to be concerned. The egg is perfectly healthy and so is the hen who laid it.
Cause of a Double Yolk
The average healthy laying hen will lay one egg every 26 hours. One embryo is supposed to be released from the ovaries as part of the egg laying process. If two embryos are released at the same time, the embryos pass through the hen's reproductive tract together and become encapsulated into the same shell rather than developing into two separate eggs. This results in a double yolked egg.
About Double Yolks
It is rare, but a single double yolked egg can produce two live chicks if fertilized and incubated properly. It is more common for both embryos to die or for only one embryo to survive.
Double yolked eggs are relatively normal and generally not considered a cause for concern. They most commonly occur in the eggs laid by young hens whose bodies have not gotten into a regular laying schedule just yet.