Assume your invoice numbers are in column A:

In an empty ("helper") column put the following formula and copy down until
the last invoice row. Change column to suit.

=IF(COUNTIF(A:A,"=" &A1)1,"Duplicate","")

Duplicate invoices will have "Duplicate" in the column. If invoice 123456
appears 3 times, each occurence will have "Duplicate" in your "helper" column

HTH

"Damondism's" wrote:

I have a spreadsheet with thousands of invoice numbers, i need to identify
duplicate numbers within a specific column. I'm not great with excel,
lay-terms would be appreciated =)