This works for me =IF(COUNTIF(C$1:C$20,A1+B1)<=1,"TRUE","FALSE")
Enter this Formula validation into Range A1:A20 and the same formula
validation for Range B1:B20
If a value is entered in A or B that totals the same as any previous total
in C it will come up with error code.
Let us know how you get on please.
Skinman