Check 2 columns of names for duplicates

[jimp]

I have 2 very large lists of names. I need to quickly see if any of the names that appear in one column match any of the names in the other column. (One list is existing customers, one list is leads for new business. I don't want to send existing customers an offer to do business.)

 

[Dave Peterson]

Chip Pearson has a bunch of techniques for working with duplicates at:
http://www.cpearson.com/excel/duplicat.htm