just add UNIQUE
`mc_userName` text NOT NULL UNIQUE,
or
CREATE TABLE `twMCUserDB` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`mc_userName` text NOT NULL,
`mc_userPass` text NOT NULL,
`tw_userName` text NOT NULL,
`tw_userPass` text NOT NULL,
PRIMARY KEY (`id`),
CONSTRAINT tb_uq UNIQUE (mc_userName)
) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1;