Three years ago we had 5 eggs under our favorite little broody Hen. She was in the nursery coop all by herself...the day after 4 of the eggs hatched I went outside to check on her and the fifth egg was zipping...2 chicks came out of that egg. One chick was larger than the other but both survived. My brother in law who worked with chickens before he retired kept saying the little one will die, no way twins will live, if it lives it will have physical/mental problems. Well 3 years later Fanny and Peanut (the little twin) are still going strong. Both are great egg layers and both are wonderful Mammas each spring. Peanut was smaller than her siblings until she was about six months old, at that point she had a growth spurt and has been the same size as the rest ever since. They aren't "identical" in looks, Peanut has more of a red tint to her feathers than Fanny does...Fanny is more of a goldish buff.
Michelle