Magento Coupon Code Validation – How to Validate

The only way to solve this problem is to either require users to create a customer account prior to using this coupon or implement custom logic to validate against billing and/or shipping information to try and accomplish the same thing. Anything else would be unreliably flakey. Checking for orders placed with the same info is already pretty flaky, complicated, but doable.

My recommendation is simply not to do this and require users to login. If you have a coupon and give the customer a message telling them they need to login or create an account to use it, they will do that to save a few bucks.

Take a look at the class Magento\SalesRule\Model\Utility located at the vendor/magento/module-sales-rule/Model/Utility.php. Coupon code validation starts in the method canProcessRule() where it can mark the rule as invalid if coupon code does not match conditions like usage per customer etc.