title: Duplicate key name 'unique_id'
body: <p>Here is the sql, however, there is an error says "<strong>*#1061 - Duplicate key name 'unique_id'*</strong> ", what is the problem.</p>
  
  <pre><code>create table `users`(
     uid int(11) auto_increment,
     unique_id varchar(23) not null unique,
     name varchar(50) not null,
     email varchar(100) not null unique,
     encrypted_password varchar(80) not null,
     salt varchar(10) not null,
     created_at datetime,
     updated_at datetime null,
    PRIMARY KEY (`unique_id`),
    UNIQUE KEY `uid` (`uid`),
    UNIQUE KEY `unique_id` (`unique_id`),
    UNIQUE KEY `email` (`email`)
  )ENGINE=InnoDB AUTO_INCREMENT=877888 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
  </code></pre>