ISA Brown lays empty micro egg. Very odd.

[SLRS]

One of our hens layed this mini egg and when I finally got around to cracking it open, it was empty...except it had an inside shell that was exactly half of the egg. The egg was about 1.1/8 inches wide and the inner shell (exactly half size) fit inside perfectly until it had dried, and I guess shrunk a little bit. Any idea what this is? It looks fake and the inner shell when placed on a flat surface is perfectly flat (like in the 3rd picture).

 

[SLRS]

No ideas?