Here are some top tips on how coupons code misuse can be prevented:
*This can be done by checking the email address or shipping address of the customer against your database of past customers. If it happens to be an existing customer, display a message on the checkout screen indicating that the referral coupon only applies to new customers and that a smaller discount amount will be applied instead.
For the Shopify platform, you can run a check against your existing customers by following the steps here.
Any usage restrictions should also be stated clearly in the referral invite emails and landing pages that customers and their friends see so that they are not surprised by it during checkout.