On Mar 20, 11:13 am, "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.

Thanks folks but I found the solution with a search in this newsgroup
- sorry if anyone has wasted any time!