Double yolk eggs are caused by the hen's internal egg laying factory not working right. It releases two yolks at the same time instead of only one. That's why it happens more often in young hens that have not got the kinks worked out yet. Some hens continue to do that when they grow up. Their internal laying factory never gets the kinks worked out. I've had that happen with a hen that I almost selected to become a member of my laying/breeding flock, but she did not make the final cut because she continued to lay a double yolker about once a week.
I suppose if you select mature hens for breeding that often lay double yolked eggs (hatching their non-double yolked ones obviously) you could increase the occurrence in your flock. Not sure why you would want to. They would be OK for frying or scrambling, but they would mess up many recipes that call for a regular egg, especially in baking. They don't hatch very well at all. Since they are extra large, you increase the chances of the hen prolapsing or getting eggbound. Usually when they lay a double yolker, they skip the next day. Not always but usually.
According to some reading I've done, feeding excess protein can cause them to lay more double yolkers too. Maybe they are being overfed.
Maybe you are too close to Key West. I've never visited but I've heard strange things have been known to happen there.
My real guess is that whoever is selling those eggs are candling their eggs, looking for blood spots, meat spots, or other internal defects and are pulling out the double yolkers to sell as novelty items. I'd think they'd have to be selecting from a whole lot of eggs to get many double yolkers though.
