why wont the formula =countif(f:f,f3)=1 not prevent a duplicate value

simmonsortho1 

Am I correct in assuming that you're trying to use this formula in a Data Validation rule of type Custom?

And that the word not is superfluous?

If so: it should work, if F3 was the active cell when you created the formula.

If you cannot solve it:

Could you attach a small sample workbook demonstrating the problem (without sensitive data), or if that is not possible, make it available through OneDrive, Google Drive, Dropbox or similar?