Happens all the time. All my hens are housed separate. I look for eggs every day, especially when I don't have a nest box in. Waiting for them to start a clutch before putting a box in. It's something I watch very closely. Didn't miss the egg. Put the bucket in the day before because her aunt started a clutch a couple days before that, so I knew it was time. Watered in the morning, no egg. checked later, two eggs. Have had clutches of +-12 eggs laid in less days many, many times. Have seen two eggs the first day many, many times. Have watered in the morning, seen an egg that wasn't there in the evening before, gone back with a nest to find two eggs. Whoever counts themselves as the experts on egg formation evidently hasn't studied the right chickens.