An unusual egg

In these times of reported shortages we are collecting plenty of eggs each day, enough still for us and for our regular clientele. Among the eggs today was a rather unusual egg similar to one laid two days ago. Over the years we have seen different deformities in chicken eggs. Tiny 'fairy' eggs, over-sized eggs, misshapen eggs and eggs with wrinkled shells. They are normally a sign of stress or are produced by an aging hen. Double yokers are not unusual too. A double yoker occurs when two ova are released and they become encapsulated in a single egg.  

The egg today was massive - 178g. When it was cracked open, inside was the expected yoke and egg white but in addition a fully formed normal sized egg with a hardened shell. The day before yesterday we had a similar one weighing 132g.

Eggs usually take 24-26 hours to form once the ovum (which becomes the yoke) is released from the ovary. On its journey through the oviduct the white of the egg is added, then further along the membrane layers are formed and then the shell, which hardens off before the egg is eventually laid. 

In this instance, and the same two days ago, I can only surmise that a normal egg was fully formed but remained in the oviduct without being laid. The next egg formation process began and caught up with the first egg and the shell was formed around the whole lot and then this went on to be laid. 

I am not sure which hen was responsible but none seem to be showing signs of trauma from the experience.