The birds haven't been laying for long, about a month, but I've had several double yolk eggs, white egg and brown egg, so I know they're from different birds. How common is a double yolk egg?
It really depends on the bird, I once had a BO that laid double yolkers several times a week the first year, then once a week the second year, and then never again.
My sons thought they were a sign of good luck.
People are often worried about them, but really does not seem to bother them or their life span, Butter was a bird that lived a long time for me.