Double Enter a bill?

I was in a tims in Banff, Alberta the other day and I bought an ice cap. I paid with a five, my dad was behinde me and his change was my marked 5. I mark all his bills. Should I mark this one?

If it's already marked, marking it again is redundant..
But if you mark the other side it will give you another shot as some people only look at one side of their bills, but then, some pay no attention to anything but the denomination.