You'll want to make sure the Attendance Limit field is set to 1 for that series - under the Booking Options section of the Series setup form (Scheduling/Retail > Series/Contracts).

You may also want to check the System Setting called 'Attendance Limit'. If this is set to 2 or more, that means the customer can book more than one spot in a class.

Please feel free to email support@zingfit.com to have the team decrease this limit to 1.

Please note- that field must be 2 or more to support guest booking!