Yes, single. That is just their typical egg. And they very quickly go from pullet sized egg to jumbo. Seemingly within days.
I've usually found double yolk eggs to be longer and extra large for the breed, you can generally spot them, but not always. Selfishly, when I notice one it usually ends up the egg I'm having for breakfast. Being the cook has to give you some advantages,
Can't say I've ever seen before about two shells seaming together. A double yolk egg is caused by two yolks releasing together as they develop and the shell is applied around both yolks with albumen added around the two.
Q: What causes double yolked eggs?
A: Egg production in the hen is controlled by the release of specific hormones, which in turn stimulate the release of a single egg yolk from the ovary. After the yolk is released from the ovary it continues its journey through the hens reproductive tract where it develops into an egg with a hard outer shell. Usually, only one yolk is released by the ovary in a given day. However, sometimes two egg yolks or on rare occasions, even three yolks may be released at the same time resulting in the formation of a double or triple yolked egg.
This release of more than one yolk at a time is due to an over stimulated ovary which occurs as a direct result of the increased level of reproductive hormones in the hen. This phenomenon appears more commonly in young hens and is also seen more frequently in meat-type strains of hens verses egg-type hens. Genetics may also be a factor involved with some hens naturally producing a higher percentage of double yolked eggs than others.