I went to a local breakfast place some years ago with two friends and we all ordered fried eggs. We each got three eggs with six yolks. The waiter said they got a whole case that way. So something caused a whole bunch of chickens all from the same place to make double yolks all at once.
You can tell a double-yoker before you crack it. The egg is slightly elongated through the middle, and has a certain appearance about it. We had chickens for years, and used to put all of the double-yokers separately in the fridge. I suspect the eggs you got were separated out the same way.