This is an old thread but the question was never answered.
A hen’s internal egg making factory has a lot of different parts. Occasionally there are some glitches. A heh is supposed to release one egg yolk a day, but occasionally some hens release two or even more instead. If they are released at the same time you will probably get a double yolked egg. If they are released at different times you can get two eggs in one day.
A hen normally makes a limited amount of shell material each day, though different hens are different. If she makes two eggs in one day, the second egg is often soft-shelled or even with no shell at all, just a membrane. She just didn’t make enough shell material for two eggs that day. The same thing can happen to color if she lays a brown egg. She may not make enough pigment for two eggs so the second egg is often lighter in color.
As long as it is not a real regular occurrence it is not a big deal. Oops happen. If this becomes the norm or spreads to other chickens you might have a disease problem, but as long as it is an occasional thing, it is no big deal.
Welcome to the forum. Hope this helps.