deleting duplicates

I have a large mailing list (A) which I want to compare to a small mailing list (B) and then exclude addresses from (A) which are already on (B). What's the best approach? Thanks!

Here is how I would do it.

Put this formula in C1 and drag down. Once the Dups are identified you can filter for Duplicates and delete those rows.

=IF(COUNTIF($A$1:$A$100,B1)>0,"duplicate","")