In this example, I had values in Column A and Column B and I wanted to identify which were duplicates.
I typed the following formula =IF(ISERROR(MATCH(A1,$B$1:$B$5,0))," ",A1) in Column D and then copied it down. This formula tells Excel to Compare Column A against Column B and to identify any values that match. Excel found that there was a match for may in Column B and there was a match for the value 2 in Column B so those are the only values that display in Column D.
If you do anything else with duplicates, let me know.
I would be glad to share it here.