I will say, though it is not common, it does happen. And usually the second egg is way smaller than the normal egg.
I have a welsummer named sprout. last summer she layed an egg at roughly 7:30pm. I watched her lay that egg on the lawn. She was still new at the laying thing. Then the next morning roughly 8:00am I let them all out and checked for eggs. There was none. Sprout went out got a drink, turned around hopped in the nest box, and 20 minutes later she had laid another full size egg. Roughly 13 hrs apart.
I have a welsummer named sprout. last summer she layed an egg at roughly 7:30pm. I watched her lay that egg on the lawn. She was still new at the laying thing. Then the next morning roughly 8:00am I let them all out and checked for eggs. There was none. Sprout went out got a drink, turned around hopped in the nest box, and 20 minutes later she had laid another full size egg. Roughly 13 hrs apart.