It seems to work ok when I tried this ..

In a new sheet,
press CTRL+A
(this selects the entire sheet)

Click Data Validation
Allow: Custom
Formula: =COUNTIF($1:$65536,A65494)<2
Click OK
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Rgds
Max
xl 97
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xdemechanik <atyahoo<dotcom
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"Louise" wrote in message
...
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