I have a large spreadsheet(7000 entries) that I would like to mark any of the
row that are duplicates. In order to match the duplicates columns A B C have
to be the same. I have attempted to use the following but it does not seem
to work:

=IF(COUNTIF(A:C,A2)1,"Duplicate","")

I place this in column D then copy it down through the spreadsheet.

Thanks for your help.