--------------
ALTER TABLE `order_items`
	ADD COLUMN `serial_id` INT(10) UNSIGNED NULL AFTER `product_id`,
	ADD KEY `idx_order_items_serial` (`serial_id`),
	ADD CONSTRAINT `fk_order_items_serial`
		FOREIGN KEY (`serial_id`)
		REFERENCES `product_serials` (`id`)
		ON DELETE SET NULL
		ON UPDATE CASCADE
--------------

ERROR 1060 (42S21) at line 51: Duplicate column name 'serial_id'