Adding a new sender email address

Adding a new sender email address

Good Morning, 

I would like to add a sender email address  ( Drflea1@gmail.com) to my account so that when send out the Save the Date eblast - it comes from his email address I run into a problem. The Notification in red appears - what does it mean and how can I make this the send email address for all outgoing communications. My email is just the registration email address.

Email Address
 
Duplicated email address found while adding your sender address.