One of my barred rocks did that awhile back. She was just finishing up a molt and hasn't done it since so I just chalked it up to a glitch in the system. No health problems or anything with her so I wouldn't worry too much about it.
My red sexlink did this once - the two on the right are hers:
What happened was she laid 2 eggs that day, one in the morning and one at night. It was just a glitch. She hasn't pulled that trick since then, and her eggs have only gotten darker!
The dark speckled egg in this pic is ALSO hers!