Add unique index on table domain_admins?

Hi, developers.

I'm using PostfixAdmin-2.3.2, the default structure of table 'domain_admins' in upgrade.php is:

    CREATE TABLE {IF_NOT_EXISTS} $domain_admins (
      `username` varchar(255) NOT NULL default '',
      `domain` varchar(255) NOT NULL default '',
      `created` datetime NOT NULL default '0000-00-00 00:00:00',
      `active` tinyint(1) NOT NULL default '1',
      KEY username (`username`)
    ) {MYISAM} COMMENT='Postfix Admin - Domain Admins';";

I was wondering why not add a unique key (or primary key) to avoid duplicate records? like this:

PRIMARY KEY (username, domain)

Otherwise we can insert multiple records like this:

INSERT INTO domain_admins (username, domain) values ('xxx@xxx', 'domain.ltd');
INSERT INTO domain_admins (username, domain) values ('xxx@xxx', 'domain.ltd');
INSERT INTO domain_admins (username, domain) values ('xxx@xxx', 'domain.ltd');

Any comments?

seems like a pretty solid idea to me.