I had the same sort of thing happen to me. Except the egg that wa inside was more like calcified. It was really hard and it didn't have a brown shell it was white. It was a little creepy
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.
Those are fascinating! Someone on here in the last few months posted that they were cracking some older eggs for their dogs, and they actually found TWO eggs inside an egg! Just fascinating!
I had one of them before. A Barred Rock laid her first one like that. It didn't have a shell though. I blame it on that it was her first egg ever and every egg I got after was normal.