My daughter found the egg, on the floor instead of in the box. She thought it was a stone -- it was a little misshapen, and about 2-3x normal size (pictures to follow). We have EEs, which have been laying eggs between large and extra large, but this one was beyond jumbo. I asked her if any of the chickens were walking funny after laying that one, but we have no clue who laid it.
I decided to blow the egg, in order to save the huge shell. I poked an extra-large hole, so that the yolk(s) could come out intact enough for us to see if it was just a double-yolker, or perhaps a triple. I blew out egg for a while, but then it wouldn't come out. The remainder rolled around in the shell, disturbingly solid-feeling. I expanded the hole a bit to feel inside... And there was another egg.
We've done some looking about on the web, and it appears that this egg-within-an-egg phenomenon is pretty rare, but not unheard of. Best theory is that an egg gets lost and backtracks up the laying mechnism, where it meets and gets engulphed by the next egg coming down.
Anyway, I'll post the photos soon, but I just had to share.
Bhadrika