On duplicate key do nothing

Question

I am inserting into the following table using LuaSQL with PtokaX API.

CREATE TABLE `requests` (
    `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `ctg` VARCHAR(15) NOT NULL,
    `msg` VARCHAR(250) NOT NULL,
    `nick` VARCHAR(32) NOT NULL,
    `filled` ENUM('Y','N') NOT NULL DEFAULT 'N',
    `dated` DATETIME NOT NULL,
    `filldate` DATETIME NULL DEFAULT NULL,
    PRIMARY KEY (`id`),
    UNIQUE INDEX `nick_msg` (`nick`, `msg`),
    UNIQUE INDEX `ctg_msg` (`ctg`, `msg`)
)
COMMENT='Requests from users in any of the categories.'
COLLATE='utf8_general_ci'
ENGINE=MyISAM;

Now, my problem is, when a user(represented by nick) tries to insert same request again, the UNIQUE index is checked and the script returns a false. This causes my script to fail and I have to restart the script.

Is there something I can do in the INSERT ... ON DUPLICATE KEY command so that it does nothing or at-least does NOT return an error in case of DUPLICATE KEY?

_{Otherwise I'd have to go for updating my dated field with the new DATETIME value.}