Maybe you can use the technique from Debra Dalgleish's site:
http://contextures.com/xlDataVal03.html

andrewmac wrote:

Hi,

I have a spreadsheet, each row of which represents an invoice from a
third party. In column A we enter the invoice number and in the
subsequent columns various information about the invoice.

I want to set a data validation so that the person entering the
invoice number is prevented from entering a number which has been used
before. Can anyone tell me the correct formula?

Many thanks,

Andrew.

--

Dave Peterson