We have such a variety of eggs on the farm. We have one hen, one of the young Cuckoo Marans ladies, who has been consistently laying HUGE eggs.
But, SURPRISE! The HUGE Cuckoo Marans eggs are double-yolkers! Two yolks in one shell! I guess she's an over-achiever! Occasionally a hen's ovulation cycle is too quick, and the result is typically a double-yolker egg. This is another example of what can occur when a young hen's egg laying track works out the kinks!
In case you missed the video showing Rebecca Bower successfully hatching two chicks from a double-yolker (a very rare event), see it here.