I would start off by greeting both of the customers. Customer and prospect equally. I would ask who arrived first and I would still help that person whether it be my current chatty customer or a new customer but I definitely would advise both that each each customer would probably take a little bit longer than something quick and easy. I would also offer both the option to reschedule and come back or set up for a different time and date but definitely you know my time is going to be offered equally to both whether it's a new customer or an existing customer.