There are 4 steps in this article section

Step 1. Select the column header of the column containing the email addresses. (or press "Ctrl+A")

Step 2. On the Home tab, in the Styles group, click Conditional Formatting. 

Step 3. Click on the "Highlight Cells Rules" and then Click on the "Duplicate Values". (This will open the "Duplicate Values dialogue box")

Step 4. Select a "formatting style", if desired, and Click OK.

You should now see the duplicate email addresses highlighted. (You may need to scroll down the sheet.)

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