Interesting and surprising. The mothers and grandmothers of Cornish X lay hatchable single-yolked eggs so it's not something bred into Cornish X. I really expected you to say she was one of those commercial hybrid egg layers that are bred for enhanced laying. But each individual hen is unique.
A hen can release two yolks in the same day. She's not supposed to but it happens. There are certain triggers that tell her to release a yolk to start the internal egg making journey through her egg making factory. If two yolks are released at the same time you can get a double yolked egg. If they are separated by a little time she might lay two eggs in the same day. Often one has a very thin shell as she does not have enough shell material for two eggs so it may not be hatchable.
I consider a double yolked egg a deformity. They don't fit in an egg carton. They are too big to be used in baking recipes. The larger size makes them a little more susceptible to egg laying issues. And they are difficult to hatch. There are some threads on here where somebody was able to get one or even two chicks from a double yolked egg but those successes usually take a lot of hands on and still often end sadly. As interesting as they are I'm not thrilled when I see a double yolked egg.
Some hens are more prone to laying double yolked eggs. It's possible it could be genetic, it may be that she was just put together wrong or her instincts that trigger the release of the egg yolks to start the egg making process are just off. That might be genetic or it might just be a glitch, something like a random birth defect. Pullets just starting to lay are more prone to egg laying mishaps of all kinds, including double yolks. Often they straighten out over time and debug their egg laying system. Your CX might or might not. One trigger that can cause them to release extra yolks is they they are eating really well, they have an excess of nutrients. I don't know if you are restricting her diet for health and longevity reasons or not.
I'm not sure what kind of breeding program you plan or what other options you have. If her eggs straighten out you can try to hatch them, single yolked eggs should hatch fine. I don't know how likely it is to be genetic. But if I had other options I personally would not use her.
