What does it mean when I get a double yolk? Is it because she didn't lay one the day before and trying to catch up. She has only been laying since the 15th of June. This was her first normal size egg, the rest have been small. I have a rooster so if it would have hatched would it have been twins? Just curious.

A double-yolker forms when one egg follows another down the shoot a little to closely; they both wrapped in the same shell. It's not uncommon to get this from time to time, although you'll find some hens who routinely produce them, it's just part of their make-up. Your hen is probably just getting up to speed, so to speak. I've often wondered about the twins thing, I suspect that since an egg will not expand to accomodate the growth of two chicks, they would begin to develop and then die.