Kidney bean shaped mass inside egg

[Chelseasducks]

I have male/female pekin ducks. I cracked open a egg had some calcium deposits and there was a hard kidney shaped mass attached to the yolk. I didn't take a photo but the mass is almost identical to this picture in an old thread that didn't have any answers https://www.backyardchickens.com/threads/odd-mass-inside-egg-picture-included.1052793/

Any ideas? The duck in question has recently stopped laying eggs too but seems fine otherwise.

 

[sourland]

Sounds as if it may be a lash egg enclosed within a normal egg.