New customer discount code, prevented to old customers

We are seeking an experienced developer to implement a new customer discount code system that restricts access solely to new clients. The design for the discount code is already finalized; however, we require assistance in ensuring that existing customers cannot exploit this offer by registering with alternative email addresses. The ideal candidate will possess a strong understanding of user authentication and code validation processes. Your expertise will be crucial in creating a seamless experience while upholding the integrity of our promotional offerings.