Add tests to check for duplicated meta records.

plugins/woocommerce/tests/php/src/Database/Migrations/CustomOrderTable/WPPostToCOTMigratorTest.php

@@ -104,6 +104,25 @@ WHERE order_id = {$order_id} AND address_type = 'shipping'
 				"
 SELECT COUNT(*) FROM {$this->data_store::get_operational_data_table_name()}
 WHERE order_id = {$order_id}
+"
+			)
+		);
+
+		$this->assertEquals(
+			1,
+			$wpdb->get_var(
+				"
+SELECT COUNT(*) FROM {$this->data_store::get_meta_table_name()}
+WHERE order_id = {$order_id} AND meta_key = 'unique_key_1'
+"
+			)
+		);
+		$this->assertEquals(
+			2,
+			$wpdb->get_var(
+				"
+SELECT COUNT(*) FROM {$this->data_store::get_meta_table_name()}
+WHERE order_id = {$order_id} AND meta_key = 'non_unique_key_1'
 "
 			)
 		);