多线程操作Mysql问题

这个存储过程，多线程下会丢失数据。

CREATE DEFINER=`root`@`%` PROCEDURE `game_register1`(IN `_status` int,IN `_account` varchar(64),IN `_password` varchar(64),IN `_platform` int)

BEGIN

	#Routine body goes here...

	DECLARE _id BIGINT;

	SET _id = -1;

	SELECT userid INTO _id FROM game_user.account WHERE account=_account AND platform=_platform;

	IF _id < 0 THEN

		INSERT INTO game_user.account(status,account,password,platform) SELECT _status,_account,_password,_platform FROM DUAL WHERE NOT EXISTS(SELECT * FROM game_user.account WHERE account=_account AND platform=_platform);

		SELECT userid INTO _id FROM game_user.account WHERE account=_account AND platform=_platform;

		IF LAST_INSERT_ID() = _id  then

			SELECT 1,_id;

		ELSE

			SELECT -2;

		END IF;

	ELSE

		SELECT -1;

	END IF;

	END

	;;

昨天经过测试，问题出在 ： INSERT INTO game_user.account(status,account,password,platform) SELECT _status,_account,_password,_platform FROM DUAL WHERE NOT EXISTS(SELECT * FROM game_user.account WHERE account=_account AND platform=_platform);
WHERE NOT EXISTS(SELECT * FROM game_user.account WHERE account=_account AND platform=_platform); 去掉这部分代码，数据不会丢失，前者数据会丢失。
不太理解为何会出现这种状况 ，希望能帮忙分析下。