Doubling has always been a problem with double guns.
The usual cause is the shooter pulling the front trigger, and under recoil his finger slipping off and inadvertently pulling the rear trigger.
This is why most shooters learned to pull the rear trigger first, and why on many custom British double guns, the rear trigger fired the more open choke first.
The second cause is a mechanical problem in the action.
Some causes are fouling, gummed up old lube, swollen wood interfering with the action, or worn parts that allow the second barrel to "jar off" under recoil.
There are several mentions in books of African hunters suffering a "double" when firing very large caliber African double rifles.
One famous case was described by Sir Stanley Baker.
He described firing his 4-bore double rifle and having it double.
The 4-bore was a black powder double rifle that fired 1/4 POUND lead slugs.
Baker was a HUGE man, but he said the recoil nearly did him in.