Twins! As someone mentioned above, it s rare to find double yolks or even blood spots when buying the massed produced eggs at supermarkets because they are inspected. If you buy local eggs or even organic eggs at the supermarket, you are more likely to have blood spots in your eggs, I guess their inspection process doesn’t catch it, but even with organic I almost never see double yolks anymore like I used to.
Blood spots are safe to eat. You can just remove it with the tip of a knife if it bothers you. When I was very young someone once told me you have to throw out the egg if it has a blood spot, but it isn’t true.
By the way, from what I understand twin chickens making it to birth/hatch is very rare. Maybe someone else will know more about that.