memberpress-use-selected-coupon-once-per-user.php

<?php
function custom_validate_coupon($errors) {
    global $wpdb;

    // If the user is not logged in, they have never used a coupon
    if(!MeprUtils::is_user_logged_in()) { return $errors; }

    $coupon_code = (isset($_POST['mepr_coupon_code']) && !empty($_POST['mepr_coupon_code'])) ? stripslashes($_POST['mepr_coupon_code']) : '';

    // Define the coupon codes here for which you want to perform the check
    $specific_coupon_codes = ['CODE1', 'CODE2', 'CODE3'];

    if(in_array($coupon_code, $specific_coupon_codes)) {
        // The coupon code is one of those specified for the check
        if(($coupon = MeprCoupon::get_one_from_code($coupon_code)) && isset($coupon->ID) && $coupon->ID) {
            $user = MeprUtils::get_currentuserinfo();
            $q = "SELECT COUNT(*) FROM {$wpdb->prefix}mepr_transactions WHERE coupon_id = {$coupon->ID} AND user_id = {$user->ID} AND status in ('complete','confirmed')";
            $count = $wpdb->get_var($q);

            if($count) {
                $errors[] = "This coupon code has already been used";
            }
        }
    }

    return $errors;
}
add_filter('mepr-validate-signup', 'custom_validate_coupon');