A few years back we began finding double and even triple yolks in many of our eggs. My wife researched it a bit and found that happens when layers are fed fertility drugs. We switched to Eggland's Best or any brand of organic eggs (whichever's on sale). We switched originally out of concern over hormones, but have since found that the eggs we eat taste better than the cheap ones.
Can't say as I care much whether the chicken's leading a full life.
When I was a kid in the ‘50s double yolkers were much more common, before the fertility drugs and antibiotics. They mostly go to restaurants. I buy eggs from a farmer in Staunton IL green eggs, brown, some white, Jumbo size and many double yolkers. His chickens scratch in the yard.