How common are double yolked eggs?

ok, everyone says its pretty common to open an egg with 2 yolks.I never done it b4. When i was young, my parents openedd 1 egg and i saw my 1st double egg. That was like 6years ago. so i saw 1 double yolked egg in 6years. How is it common? Or am i just unlucky, or do u have to buy a certain brand of eggs. I buy white eggs that probably came from a factory, are factory eggs and natural growth eggs the same when producing double yolks?