How would I be able to duplicate my auto_increment? DATABASE PHP

How would I be able to duplicate my auto_increment? DATABASE PHP

Hey how would I be able to duplicate my only auto increment key to another
key, basically I want my (' id ') to display the same information on my ('
user_id '), here is the code:
CREATE TABLE IF NOT EXISTS `".$db_table_prefix."users` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(10) NOT NULL,
`user_name` varchar(50) NOT NULL,
`username` varchar(50) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`id`)
How would I be able to input the same information from my id to my user_id?