For alert only you could use Conditional Formatting to highlight duplicated
numbers.

Both instances will be highlighted.

In FormatCFFormula is: =COUNTIF($A$1:$A$50,A1)<1

Format to a color and OK

If you wanted to PREVENT duplicates you would enter this formula in Data
ValidationAllowCustom

=COUNTIF($A$1:$A$50,A1)=1


Gord Dibben MS Excel MVP

On Wed, 27 May 2009 14:36:11 -0700, KKD
wrote:

I have rows of data that contain a column with an invoice number, if I am
entering a duplicate invoice number, I want to be alerted. I want to be
allowed to enter duplicates, but I want Excel to somehow alert me that it is
a duplicate so I can check it. What is a good way to do this?