Exclude Dupllicate

[JJ]

Can anyone tell me if it is possible to format a colomn of cell to prevent
duplicate information from being entered. For example, I do not want to
enter the same invoice number twice.

Thanks

 

[Don Pistulka]

JJ,

Set up a data validation range. Select the range a1:a10. In the validation
settings set "Allow" equal to "Custom", type in this fomula:

=countif($A$1:$A$10,A1)=1

Don Pistulka