Bug in core storage Duplicate entry '<uid>-<hash>' for key 'PRIMARY'

CREATE TABLE `users_cookie` (
`uid` int(11) NOT NULL,
`u_cookie` varchar(32) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
`u_cookie_time` int(11) NOT NULL,
`is_ban` tinyint(1) NOT NULL DEFAULT '0',
`protect_L3` tinyint(1) NOT NULL DEFAULT '0',
PRIMARY KEY (`uid`,`u_cookie`),
KEY `u_cookie` (`u_cookie`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
/*!50100 PARTITION BY KEY (uid)
PARTITIONS 6 */;

Reindex innodb Index (upgrade mariadb 5.3 to 5.5.28a)
[SQL] alter table bb_users_cook engine = innodb;
[Err] 1062 - Duplicate entry '<uid>-<hash>' for key 'PRIMARY'

i test table and not finded dublicate
mariadb 5.3 good work