My mom called last night to tell me she'd peeled a hard boiled egg and it had two yolks. (That's her egg in the picture below.)
I've never seen a double yolk egg myself and I was curious how common it is so I decided to do a little research on the topic.
I found an explanation of the double yolk phenomenon:
Double Yolkers appear when ovulation occurs too rapidly, or when one yolk somehow gets "lost" and is joined by the next yolk. Double yolkers may be by a pullet whose productive cycle is not yet well synchronized. They're occasionally laid by a heavy-breed hen, often as an inherited trait.
On that site you'll also find descriptions of other egg oddities. Who knew you could have no yolkers, double shelled eggs, eggs without shells, spoon shaped eggs, eggs with tails, wormy eggs, triple yolkers, and even a nine yolker? Yikes!
Throughout history and in different cultures, finding a double yolk has been considered to mean anything from an impending wedding to a financial windfall to a death in the family.
So would a double yolked egg produce twins? Probably not:
... the presence of the two yolks would generally lead to an unsuccessful hatch. The nutritional value of the egg white, which is an important food supply for the embryo is insufficient for the two embyros.