When booking for someone who already has an account in BookingLive please ensure you select Buying for someone else, then use the Select participant search box to select the relevant existing participant.
If the email is manually typed the "Participants details have already been used, please enter new details" error message will show as a duplicate user account is trying to be created.
This error message may also show if a user is already booked onto the Event or in the Waiting List.