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)