In general, the yolks in double yolk eggs are smaller but the eggs overall are bigger.
About 3 to 5 percent of hens’ eggs have two yolks. Some hens, driven by their genes and the conformation of their oviducts, seem to specialize in producing these twins.
When you find a double yolker in an otherwise normal dozen, it’s a mistake, not of the hen, but of the inspectors at the egg farm, because all eggs are candled before being sold, and candling will show up any double-yolk eggs, which are put aside for special uses.
There’s nothing wrong with eating a double-yolk egg when you come across one. In fact, double yolkers are in demand for their novelty, and the supply can’t keep up.
When you do come across one, fry it sunny sides up.