Update: I have entered a formula of =(countif(A:AA,A1)<2).
This works but only if the duplicate occurs in the same column. If I enter
24 in column A and 24 in Column D, it doesn't pick them up.

What am I doing wrong???

Thank you.

Louise

"Louise" wrote:

How do I set a formula in Excel so it will not allow me to enter duplicate
values on a worksheet? I have used the Data Validation tool before to set
the countif function on a particular column but how do I set it for the whole
worksheet?

Thank you.

Louise