RomikChef не знаешь - не говори.
в MySQL - не обязательно 

mysql> create table test(
    -> id int auto_increment,
    -> index(id));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into test values
    -> (1),(2),(1);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> select * from test;
+----+
| id |
+----+
|  1 |
|  1 |
|  2 |
+----+
3 rows in set (0.01 sec)