Double Yolks

I have had about 3 double yolks when my 2 hens first started to lay (they where in the first couple of weeks of small eggs)

does anyone know what causes double yolks or when/why a hen is most likly to produce them??

Obviously this is only if it will have no negative impact on the hens.

Thanks

It's just like twins, they release two yolks from the ovary at the same time but unlike twins they wrap them both up in the same egg.