Egg with double shell

It was a very strange thing, when I grabbed the eggs's box to start cooking.

When I took the last egg from the box, I was like Ahhh?

It had a double shell I have never seen before....and how the egg got it. I couldn't find the answer. I put it back to refregerator, after checking it from all the sides and taken pics... it is still there.

The only option can be there was a shell from broken or cooked egg left in the egg's box what is strange and unusual. If so...how that egg got placed into that shell, not being notices, and fits perfectly into it? It was kind of funky and yes, it is funky and kind of funny... unless it is a mistery of the nature.

It is probably a video time soon for discovery :) Just curious.