Double Yolks?

[shesham]

I am curious. How common are double yolks? My son just ate two of them this morning. A couple of our girls have only just started laying this past week at 18 weeks. Is it just something that happens when they are starting out, or can we expect more doubles because of genetics?
We have 3 RIR and 5 black sex-link

 

[CoyoteMagic]

My RIR and Black sexies have consistanly laid doubles 1-2 times a week for the last 2 years. My Red sexies don't seem to lay them that often

 

[chickenfever]

My newly laying pullets only laid double yolkers for the first couple of months.