This rule usually means a spanking outside of the home (usually at school)  earns a further punishment at home – this rule can also work in reverse.

Terry's mother, of the authors Rose and Terry, explains her implementation of the Double Punishment Rule: “We have a house rule that when Terry gets paddled or spanked at school he gets spanked twice at home. The first time is immediately after school and again two days later so he will have a long memory of the event that caused his pain. It doesn’t make any difference if he gets one swat or a can’t-sit-down-blistering at school, he gets the hairbrush when he gets home. And when I spank, he can never sit down for quite a while.

If Father O’Brian had given him 50 swats and he came home with a fire that just wouldn’t quit, he would still get the hairbrush. If he gets the brush at home and earns a real blistering at school, I expect it to be just as hard and long as if you were starting with an un-spanked bottom.”