One egg, two yolks
The world is a terrifying and uncertain place as of late. When nature gifts us with small miracles, we should take a moment to pause and appreciate them. They may come in the form of a post-storm rainbow, the blur of a hummingbird's wings, or the soft mew of a newborn kitten. One could also manifest in your very own kitchen in the form of a double-yolked egg. That's dandy because after you're finished admiring the wonder and majesty of the universe and contemplating the existence of a supreme deity, you can make yourself a kick-ass and extremely Instagrammable breakfast.
Double-yolked eggs are produced when a hen's oviduct—that's the tube through which eggs are passed from the ovary—releases two yolks in quick succession. That's often due to a hormonal imbalance or change, especially when a young chicken is are beginning her egg production, or when an older hen is winding that process down. The two yolks travel along together and are eventually housed within the same shell, which tends to be larger than that of a conventional egg. It happens around once in every thousand eggs laid by an average hen, but some breeds of chickens (including heavier breeds like the Buff Orpington, as well as the Black Sex Link and the Production Red which are both specifically bred for high production) are more predisposed to lay double-yolkers.
Though a standard carton of grocery store eggs may on occasion contain a double yolk, they're most often discovered at the egg production facility by a process called "candling." This involves passing the egg in front of a bright light to show what's going on inside so that it can be properly graded for the commercial market. These eggs won't generally show up on conventional supermarket shelves—though it certainly happens from time to time—but will show up more frequently in eggs that come directly from farmers. Some smaller producers specialize in double-yolked eggs and will remove them from the regular production line to include in specifically labeled all-double-yolks cartons.
Double-yolked eggs carry a certain amount of spiritual weight. In many cultures, they're a symbol of fertility (possibly twins) and general good fortune. But woe be unto the unfortunate ancient Norse soul who came across a double-yolker, for a family member was about to meet their bitter end. And for you, modern citizen of the world, it means only that you have been given a rare chance to marvel at a lovely hiccup of nature—and have a gloriously extra-oozy breakfast.