When my bluebelle started laying she went haywire for a couple of months, and would regularly lay two or three eggs a day (usually two soft shelled). One day she laid a hard shell, a soft shell, and a hard shell double yolker! Our vet didn't believe me, but I saw her lay each one with my very own eyes.
She's into her second season now, and has settled down to laying one a day, regular as clockwork. No double yolkers either.