This Error can be caused when the registrant already has a registration for the event. Instead, you will want to edit the existing registration with the following steps. 
  1. Go to the registrant tab of your event
  2. In the search for the name of the host/ registrant 
  3. Hit the expand button next to their name
  4. Click edit
  5. Click edit registration 
  6. Use the table to add your registration options
  7. Click save