Added primary key to coupon lookup.

Otherwise REPLACE query adds a multiple identical rows.

plugins/woocommerce-admin/includes/class-wc-admin-api-init.php

@@ -342,7 +342,7 @@ class WC_Admin_Api_Init {
 			coupon_id BIGINT UNSIGNED NOT NULL,
 			date_created timestamp DEFAULT '0000-00-00 00:00:00' NOT NULL,
 			coupon_gross_discount double DEFAULT 0 NOT NULL,
-			KEY order_id (order_id),
+			PRIMARY KEY (order_id, coupon_id),
 			KEY coupon_id (coupon_id),
 			KEY date_created (date_created)
 		  ) $collate;