Chip Pearson has lots of techniques to work with duplicates:
http://www.cpearson.com/excel/duplicat.htm

Julie - Excel beginner wrote:

Hi everyone

This is my first post. I have hunted in the history, but cannot find
the answer to my question - I hope someone can help.

I have 2 files - one from my CRM system of people I am planning to
email
one from my email marketing system of people who
have unsubscribed

both in Excel 2003.

The common point is email addresses, and I want to look for email
addresses that occur in both lists. These will be unsubscribers and I
don't want to mail them any more.

Can I compare the 2 lists and highlight email addresses that occur in
both, so that I can then delete them from the sending list?

Many thanks for your help! (If you need any help with your electrics,
UK based, let me know!)

Julie

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Dave Peterson