yes that is what they use. many years ago a friend of mine lived on an egg farm and they had an xray machine that every egg went thru and the double yolkers got picked out coz they were worth more money. it may be more sophisticated now though.
Boring bit - double yolk eggs most often come from immature birds who haven't got their hormones in order for regular laying. The eggs will always be larger than 'normal' for the breed, so if you want to aim for them - unless they are sold as guaranteed double - buy the 'large' size and you might get lucky..
Three or four yolk eggs also turn up as well.....isn't nature wonderful!
..I used to be a breakfast chef and our eggs would come on trays of 36.... one morning almost all of the eggs from one tray all turned out to be double yolkers.... we had happy customers that day....
I used to work in a super market, we got an egg delivery every day and each order had a box of double yolkers, I think in the 3 years I worked there we had about 10 boxes. The delivery driver said there were never that many of them.