Sometimes things go awry in the reproductive tract and it will try to lay down a second shell on an egg. Sometimes it is even a second hard shell over a shelled egg. Generally it is nothing to worry about.
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Looks like the egg backed up in the plumbing and got a membrane put on it.
How are the pullets/hens behaving? Anyone seem off?
All sorts of things can have an impact on how eggs turn out as other greeters have said. Here's an interesting article about some of the causes that may be of interest to you: Common egg quality problems