Check this out!! Routine collection of eggs today and there it was. My poor hen, that must have been like pushing out a 15 pound baby. But I couldn't see any of them limping so.... no stiches required! (or intact perineum as we say in the midwifing business - hmmm, do chickens have perineums?).
Anyway, I just had to crack it open and sure enough it was a double yoker. These do not normally develop, but I have heard a few stories of twin chickens. Check this link out for an assisted twin chicken birth. In this eggs case, even it had been fertile (I have no fully grown roosters at present), it wouldn't have survived as the yolks are partially joined.
Because I don't normally have roosters, when I want more chooks I buy in fertile eggs and put them under a broody hen. All going well, 21 days later I have cute fluffy little babies.
I often buy a mix of purebred eggs. This can be a great way to try out different breeds and is often a good way to get expensive breeds cheap. How? Well, breeders usually sell eggs in dozens or half dozens but if they have some extras they make up mixes and sell these at the same price as the cheapest breed in the mix. The only problem is telling who is who when they hatch. But I found this page which includes pictures of different breed chicks - very handy!