ALTER TABLE .. ADD PRIMARY KEY IF NOT EXISTS -> ERROR 1068 (42000): Multiple primary key defined

MariaDB [test]> CREATE TABLE `market_deductions` (

    ->   `event_id` bigint(20) unsigned NOT NULL DEFAULT '0',

    ->   `market_id` bigint(20) unsigned NOT NULL DEFAULT '0',

    ->   PRIMARY KEY (`event_id`,`market_id`)

    -> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC;

Query OK, 0 rows affected (0.01 sec)

MariaDB [test]>  ALTER TABLE market_deductions ADD PRIMARY KEY IF NOT EXISTS event_id (event_id,market_id);

ERROR 1068 (42000): Multiple primary key defined