weird but cute!! - Is this common?

The other day I hard boiled a few eggs and one had a hard piece inside, I opened it up to find another little egg - really cute!!
The little egg had no yolk, but the large egg did.  Couple of piccies below!
Is this common, has anyone else had one?

Not common, and I can't remember why it happens, but interesting!  It's not a double yolker, its a double egger!!   

I have read that it happens when a normal egg disturbed in the shell gland could move back into the oviduct and start production all over