I'm Trying to use COUNTIF in data validation (custom) to check that a number entered hasn't already been used further up the same column. I have =COUNTIF(F:F,F5)=1 which will trigger an error alert if a duplicate number is used and it works fine.
I want to keep it simple which is why I'm trying to do it without a macro, but how can I copy this down all cells in the column without having to retype it for every one?