sjp1966, I am confused because from your request the formula should work. All duplicates that come out in Cell C are those entries that are 304 names while all NO that it spills out are those part of 923 names in Column A that are not in Column B.

Note, the B2 portion of the Formula above looks at Column A and gives result.

OR since Column A is 923 lines and Column B is 304, use as below and copy down.

=IF(COUNTIF($B:$B,A2)>=1,"Duplicate","No")

hope this works. All duplicate are 304 and all No are others...