How rare is this?

Eggcellent!

There is a farm near me that sells double-yolked eggs. So with - some planning - I assume not so rare :-) Still looks yummy!

It’s more likely to get multiple eggs with double yolks than you think

1 in 400 million, I just know this

Happens about once a month or so with my chickens then again some of my chickens are quite a few years old and it gets more common as things 'wear in' with age.

In a shop close to my old house, a store used to sell these kinds of eggs and, for the most part, they did have 2 yolks