Ah.. I would say common in new layers.. and a major undesirable reproductive hiccup if a mature layer.
Talk about BAD luck of the draw. Too many birds lose their life to egg binding which is often (not always) cause by multi yolkers.
The maximum I have read about was alleged to be a 9 yolk chicken egg..
I have gotten several excessively large eggs, usually from pullets or those returning to lay from brooding or molt... I think my largest chicken egg was 104 grams.. cringe worthy and beautiful, losing hens outweighs my intrigue and curbs my excitement to see more.
So you get a whole extra egg one day and it's a double yolker? How many birds of what type and how many eggs is your norm?