Insert to database without duplicate in php

How can I insert data to database without duplicate of the name for example if the name found in database show message how can do this??

<?php
$username ="root";
$password ="";
$hostname="localhost";
$db="a";
$dbhandle=mysql_connect($hostname ,$username ,$password,$db)or die('not connect to the database because:'.mysql_error());
 mysql_select_db($db,$dbhandle);
 $myusername=$_POST['user'];
$mypassword=$_POST['pass'];
$mypassword_conf=$_POST['Password_conff'];
if($mypassword==$mypassword_conf)
{
$sql="INSERT INTO aa( username, password,pass_con) VALUES 
('$myusername','$mypassword','$mypassword_conf')";
   if(! mysql_query($sql,$dbhandle))
      echo "not insert";
      else
         echo "insert is Done";

          mysql_close();

           }
else
{
    echo "not insert to db found error";
}

?>