er, well, if the machinery happens to misfire too often that would certainly self-select that line of inquiry into reproductive success kinda moot...A hen's internal egg making factory is pretty complex, a lot of different parts that have to be right for the egg to be right. That's not just putting the egg together but figuring out when to lay and where to lay. The more I learn about that the more amazed I am that so many gt it right to start with.
One part of that process is releasing the yolk to make an egg. If they do it right they release on yolk a day at the right time that the egg can be laid during daylight hours. But occasionally they might release two or more yolks in one day. If they are spread out a hen can actually lay two eggs in one day, though the second one is often really thin-shelled or even shell-less. If they are released at the same time you can get a double-yolker. These are a lot more common in pullets just beginning to lay and usually they get the bugs out of their internal factory pretty quickly. But any hen no matter how old she is can have a glitch. I figure we're all entitled to an occasionally oops as long as it doesn't become the norm.