Constraint allowing value up to 2 times but no more than 2 times

I'm trying to come up with a way to allow one column to have the same value up to two times, but no more than two times. I attempted adding a constraint to check the count of the value in the field  - count (trsqqq) <=2 but aggregate functions are not allowed in constraints. Is there another way to do this? Any help would be greatly appreciated.