alter ignore table yourTableName add constraint unique key(yourColumName);

mysql> create table RemoveDuplicateEntry
   −> (
   −> Id int,
   −> Name varchar(100)
   −> )ENGINE = MyISAM;
Query OK, 0 rows affected (0.15 sec)

mysql> alter table RemoveDuplicateEntry add constraint unique key(Id);
Query OK, 0 rows affected (0.56 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc RemoveDuplicateEntry;

+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| Id    | int(11)      | YES  | UNI | NULL    |       |
| Name  | varchar(100) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into RemoveDuplicateEntry values(101,'John');
Query OK, 1 row affected (0.06 sec)

mysql> insert into RemoveDuplicateEntry values(101,'John');
ERROR 1062 (23000): Duplicate entry '101' for key 'Id'

mysql> select *from RemoveDuplicateEntry;

+------+------+
| Id   | Name |
+------+------+
| 101  | John |
+------+------+
1 row in set (0.00 sec)