This message occurs when the constituent already has this email address on their record. To confirm this, follow the below steps:
  1. Open the constituent record
  2. Click on the Contact tab
  3. Underneath Email Addresses, verify the email address is listed here

If you need to make an email address primary, follow the below steps:
  1. Click drop down next to email address
  2. Select Edit
  3. Check Set as primary email address
  4. Save