MemberPress: Only Allow Coupon Once Per User

function custom_validate_coupon( $errors ) {

	global $wpdb;

	//If user is not logged in, then they've never used a coupon before

	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'] ):'';

	if( ( $coupon = MeprCoupon::get_one_from_code( $coupon_code ) ) && isset( $coupon->ID ) && $coupon->ID ) {

		$user = MeprUtils::get_currentuserinfo();

		$query = "SELECT COUNT(*) FROM {$wpdb->prefix}mepr_transactions WHERE coupon_id = {$coupon->ID} AND user_id = {$user->ID} AND status in ('complete','confirmed')";

		$count = $wpdb->get_var( $query );

		if( $count ) {

			$errors[] = 'This coupon code has already been used';

		}

	}

	return $errors;

}

add_filter('mepr-validate-signup', 'custom_validate_coupon');