Index on Primary Key: is there any reason for that? I'm auditing the schema of a MySQL database used by a off-the-shelf e-commerce application and I found this as output of SHOW CREATE TABLE thistable:

CREATE TABLE `thistable` 
(
    `case_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `company_id` int(11) NOT NULL DEFAULT '0',
    (...)
    `timestamp` int(11) unsigned NOT NULL DEFAULT '0',
    PRIMARY KEY (`case_id`),
    KEY `idx_case_id` (`case_id`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=21688281 DEFAULT CHARSET=utf8


It explicitly adds an index on a primary key. 

Is this a blunder of the database designer, or is there actually a reason why is that? 

It is worth noting that this table was MyISAM in origin.
No, there is clearly no good reason.

Most probably the index originally contained another column that has been dropped - at least, this is the only explanation from the top of my mind.