Problem with ON DUPLICATE KEY UPDATE

I have a table defined like this:

CREATE TABLE `sync_orders_attempts` (
	`attempt_id` INT(11) NOT NULL AUTO_INCREMENT,
	`incremental_id` INT(11) NULL DEFAULT NULL,
	`order_id` INT(11) NULL DEFAULT NULL,
	`attempt_time` DATETIME NULL DEFAULT NULL,
	`raw_response` TEXT NULL,
	`update_response` TEXT NULL,
	`soap_data` VARCHAR(5000) NULL DEFAULT NULL,
	`update_data` TEXT NULL,
	INDEX `Index 1` (`attempt_id`, `incremental_id`, `order_id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=MyISAM
ROW_FORMAT=DEFAULT

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When running an insert on duplicate key update query, it is ignoring the duplicate key.

INSERT INTO SYNC_ORDERS_ATTEMPTS (`incremental_id`, 
                                  `order_id`, 
                                  `raw_response`, 
                                  `update_response`, 
                                  `soap_data`, 
                                  `update_data`,  
                                  `attempt_time`) 
                          VALUES (0,              
                                  0,
                                  'XXX',
                                  'XXXXXX',
                                  'XXXXXXX',
                                  'XXXXXXXXXX',
                                  now())
ON DUPLICATE KEY UPDATE `incremental_id`  = VALUES(`incremental_id`), 
                        `order_id`        = VALUES(`order_id`), 
                        `raw_response`    = VALUES(`raw_response`), 
			`update_response` = VALUES(`update_response`), 
                        `soap_data`       = VALUES(`soap_data`), 
			`update_data`     = VALUES(`update_data`), 
			`attempt_time`    = VALUES(`attempt_time`)

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If I remove the autoinc from the attempt_id column and the index, it still ignores the duplicate key.
How do I fix this?