There are some people who will tell you otherwise but chickens CAN lay two eggs in a day. It is not the same as double yolks and happens rarely but it DOES happen. I wouldn't worry about it because it likely won't continue.
We have 5 pullets and we just got our first eggs today, 1 lone green/blue that came from one of the EE's maybe and then I went back out and there were 13 white eggs piled up in a corner where I had previously looked and all was there was our white pullet Fancy. There were no eggs under that I could see and I have not seen any there until today.
I have one black star who started laying very sporadically 3 weeks ago. She has since laid 2 eggs at a time on 3 different occasions. I have been in the coop with her or just outside 2 of the times so I know she did it. No one else has started laying yet and her eggs are still little.
What time are you checking for eggs? Sometimes I get 4 in a day from three chooks, but it's because I checked around 3pm or so the day before and another was laid between then and the chooks bedtime. I also got two in one hour from the same chook once, and I suspect the first egg got held up and stayed in the canal somewhere until the second egg was ready lay, so the second one gave the "stuck" one a push to the exit. LOL
This has also been happening to me. I get 6 good eggs and one egg that has no shell about every rhird day. This egg has a membrain like covering that will allow you to pick it if you are carefull. Any Ideas?