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