mysql> create table trial (id varchar(5) primary key, perl_mod varchar(30), summary varchar(255));
Query OK, 0 rows affected (0.06 sec)

mysql> insert into trial set id='aaaaa',
> perl_mod='CGI',
> summary='Essential';
Query OK, 1 row affected (0.04 sec)

mysql> insert into trial set id='aaaab',
> perl_mod='DBI',
> summary='Essential';
Query OK, 1 row affected (0.00 sec)

mysql> insert into trial set id='aaaab',
> perl_mod='Lingua__Romana__Perligata',
> summary='Fun';
ERROR 1062: Duplicate entry 'aaaab' for key 1
mysql>