How to prevent duplicate usernames when people register? PHP/MySQL

$connect = mysql_connect('whatever','whatever','whatever');
$database = mysql_select_db('your dbname');
$username = $_POST['username'];
if(isset($username)){
$mysql_get_users = mysql_query("SELECT * FROM table_name where username='$username'");

$get_rows = mysql_affected_rows($connect);

if($get_rows >=1){
echo "user exists";
die();
}

else{
echo "user do not exists";
}
}