Certain provides the ability to prevent multiple registrations and profiles with the same email address. The error "A registration already exists with this email address. Please log in to your existing registration rather than creating a new registration." indicates that the email address entered by a registrant is already being used by a different registration, at either the account or event level. If a registrant encounters this error, you need to verify the email address being used, and the form's settings to enforce email uniqueness within the event or on the account.

Check your form email uniqueness setting

• Navigate to Plan > Forms > edit a form
• Select Registrant Details then scroll down to Other Options

Review and select the appropriate email uniqueness configuration

• Do not enforce email uniqueness 
  • • Allows registrants to use any email address whether existing on another registration or not. No error will be posted if the same email address is used in more than one registration, but it will allow for duplicate registrations.
• Enforce email uniqueness within this event
  • • Prevents duplicate registrations within the specific event. This will also trigger the error message if an attendee attempts to register using an email address that already exists for another registration within the event.
• Enforce email uniqueness within this account
  • • Prevents duplicate profiles from being created with the same email address. This will also trigger the error message if an attendee attempts to register using an email address that already exists for another profile within the account, whether for that specific event, or any other. 

The best practice when enforcing email uniqueness is to add an entry section to your form to collect the attendee's "username and password" or their "last name and email address". If the attendee receives "A registration already exists with this email address" error, advise them to log in using their existing email address and password.