This indicates that the customer's email address is on the system.  If this is the case, you will not need to add customer on the System.  Instead use the Search / Look up page "How do I search for a customer on the Search / Lookup a customer? " and search for the customer using the customer's email address that you were trying to enter.

In the Search Results, click on the customers name to open up the Customer Details Page of that specific customer.

If the message appears, it means at some point we had added the customer into the system with her email address.