Double-Yolked Eggs?

I have four young hens, all of which started laying during the month of July. I have two barred rocks, one rhode island red, and one danish brown leghorn. My leghorn (Bob)'s first egg was a double-yolked egg, which I heard isn't all that uncommon. However, since July, we've gotten about five double-yolked eggs, all from different hens.

Although it is pretty cool to get the really big eggs, I'm just worried about my hens' health.. If it was just one hen, I wouldn't be so concerned, but I'm worried that it might be something I'm giving them? I'm not sure if that would affect their laying habits. They always have organic lay pellets and water available to them, and every day I give them an ear of corn or some bananas (or another special treat if I have anything) and some sprouts if I have any available. Their coop is NOT ideal... we are in the process of building them a new one, but the one they have is one of those kits that you can construct in an hour. They definitely need more space, and I would let them out during the day but we are having landscaping done.

Anyway, it may just be that their bodies are not quite used to laying yet. Still, I just want to make sure that nothing I am doing is stressing them out and messing with their hormones. Any advice would be welcome!