Maybe you could use conditional formatting. If your inputs are A1:A10 select
A1:A10, Format/Conditional formatting

Formula Is: =COUNTIF(A$1:A1,A1)1

and select a cell or font format.


"Todd" wrote:

I'm entering invoice identification numbers and want to format the column so
that if a duplicate number is entered by mistake it will be flagged as an
error. I'm using Excel 2002. Thank you in advance for any help.