According to the web site Fresh Eggs Daily, this is an ability for which some countries/consumers specifically breed their chickens. This talent for double-yolk eggs is known to be common for heavier-laying breeds and hybrids, so having a Leghorn that does so is not a surprise. Additionally, this can happen to young layers or hens who are tailing off in their laying life, regardless of breed.
Unfortunately, for many reasons, this kind of talented chicken can suffer health issues from becoming egg-bound (what killed my pet snake) and vent prolapse. Thankfully, I know which chicken is doing this. So I can pay her a little more attention and make sure she is healthy and all systems are still laying.