We had one 3 years ago. I wish I would have had the sense to get a picture. The inside egg was a perfectly normal xLarge egg. The other egg contained the inside egg, lots of egg white, and its own yolk. The thing was HUGE. Over 2.5 inches thick at the middle. I have no idea how she managed to lay it.
The Barred Rock hen that laid it didn't lay again for 3 weeks. I watched her closely, because I was afraid she may have been hurt by the experience. She is still here, but has never repeated the event. She just lays her normal xLarge-Jumbo egg.
Matt