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Yes double yolkers are out there. When they hatch usually one lives the other dies. A lady on here helped her double yolkers out and they survived. I don't know if they are actual twins or more like fraternal twins.
Fraternal twins are real twins too!
But they'd be fraternal, if they were identical then they'd probably be sharing a yolk, like human identicals often share placentas. Plus, each yolk develops it's own germinal disc (the big cell you can see on yolks, it's where the fetus will grow from) prior to fertilization. A yolk with 2 disks winding up in an egg with a yolk with one disc and it transplanting seems pretty unlikely too, so I guess they're probably always fraternal. If an egg did have two, the chicks probably wouldn't make it because they'd have no foods
What if in early stage of development the yolk/original cell split in two and then had identical chickens, though they would probably not live cause they would be splitting the yolk 50/50