Yeah I've only bought them once and it was unintentional. Living in a place where I didn't fully grasp thr language, I thought I was having the luckiest egg day ever until I translated the carton!
They do sell double-yolkers as a thing. You can shine a light through the shell to identify them, apparently. Higher value product so producers who get enough of them will go to the extra trouble of sorting them.
It's a genetic trait, so if they kept breeding a twin-maker hen for efficiency alone (if you raise chicks, you get 2 for 1 effectively), that could mean that most of their stock are now laying dual yolks.