I once had a full dozen double-yolks. I emailed the company Grayridge Farms in Ontario, Canada. The representative said it had something to do with the timing of the egg lay. I think (this was a couple of years ago) she said early in the fertile cycle = double-yolk?! I too thought genetic mutations caused by hormone additives was the cause; however, I was informed that I had simply missed out on double the protein (I had thrown them all out).