mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.13    |
+-----------+
1 row in set (0,00 sec)
mysql> create table testuniq (id int auto_increment primary key, value int unique not null) engine=innodb;
Query OK, 0 rows affected (0,01 sec)

mysql> alter table testuniq disable keys;
Query OK, 0 rows affected, 1 warning (0,00 sec)

mysql> show warnings;
+-------+------+--------------------------------------------------------------+
| Level | Code | Message                                                      |
+-------+------+--------------------------------------------------------------+
| Note  | 1031 | Table storage engine for 'testuniq' doesn't have this option |
+-------+------+--------------------------------------------------------------+
1 row in set (0,00 sec)

mysql> create table testuniq (id int auto_increment primary key, value int unique not null) engine=myisam;
Query OK, 0 rows affected (0,00 sec)

mysql> alter table testuniq disable keys;
Query OK, 0 rows affected (0,00 sec)

mysql> insert into testuniq values (1,1);
Query OK, 1 row affected (0,00 sec)

mysql> insert into testuniq values (1,1);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into testuniq values (2,1);
ERROR 1062 (23000): Duplicate entry '1' for key 'value'