That is kind of what I was thinking too, where two eggs got packed in there together! They are moulting or at least probably would have been when the egg had been laid.
It definitely is a little freaky!! This was from an older chicken, at least a couple of years old.
The egg was very big and had a yolk in there as well as this.
My sister had this in her egg today - the eggs are from my own flock! Was this from a second egg being formed? Has anyone come across this before? This is the first time I have encountered this!