UNIQUE Column detects 'u' = 'ú'

[Fabricio Martinez]

I have this table:
CREATE TABLE `data` (
`id_data` int(10) unsigned NOT NULL auto_increment,
`data` varchar(255) character set utf8 collate utf8_spanish_ci NOT NULL default '',
`source` varchar(255) character set utf8 collate utf8_spanish_ci NOT NULL default '',
PRIMARY KEY (`id_data`),
UNIQUE KEY `U_DATA` (`data`),
KEY `I_DATA` (`data`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci;

and for some reason Im getting duplicate entry errors for entering 'Cu' and 'Cú'
How can I fix this?