Three indexes on the same column will be created! If seen this in production databases, and it just makes things slow and uses extra storage. When creating DDL, give indexes a name, then this cannot happen.
MariaDB [tt]> show create table bla_di_da\G
*************************** 1. row ***************************
Table: bla_di_da
Create Table: CREATE TABLE `bla_di_da` (
`bla` int(11) DEFAULT NULL,
`di` int(11) DEFAULT NULL,
`da` int(11) DEFAULT NULL,
KEY `di` (`di`),
KEY `di_2` (`di`),
KEY `di_3` (`di`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row in set (0.00 sec)