CREATE IGNORE INDEX

mysql> create table test(id int);
Query OK, 0 rows affected (0.35 sec)

mysql> insert into test
    -> select 1 union all
    -> select 2 union all
    -> select 3 union all
    -> select 2 union all
    -> select 1 union all
    -> select 4;
Query OK, 6 rows affected (0.21 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> select * from test;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    2 |
|    1 |
|    4 |
+------+
6 rows in set (0.00 sec)

mysql> alter ignore table test add unique (id);
Query OK, 6 rows affected (2.06 sec)
Records: 6  Duplicates: 2  Warnings: 0

mysql> select * from test;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
+------+
4 rows in set (0.00 sec)

mysql>