1. Insert a column between Column A & Column B
2. Type ( or cut & paste) the following formula in cell B1:
=IF(ISERROR(MATCH(A1,$C$1:$C$150,0)),"","Match in Col C")
3. Select cells B1:B150
4. Then "Edit", select "Fill" on the Edit menu, and then click “Down”.
This will compare Column A against Column C the only difference is that instead of copying the contents of the matching cell I insert the comment "Match in Col C" next to the name in Column A matched in Column C.
The columns do not have to be sorted, although it might be easier.
I Started at Row 1, change the formula if needed.
I used only 150 rows to check, again change if needed.
I inserted a column, because I find it makes it visually easier to determine the outcome.