Hi Steve, This formula should help you. Create a third column and insert formula in C2 assuming A1 and B1 are titles and drag down or double click small + sign on right end of the formula cell to paste down . What the formula below is saying is, look at Column A and compare with individual cell in copy B, if any duplicate "Duplicate" and if not "No". I hope this helps.

=IF(COUNTIF($A:$A,B2)>=1,"Duplicate","No")