Help :P

//-Form Post-//

<form method='post'>
<?php
if(isset($_POST['submit'])){
	$username = $_POST['username'];
	$password = md5($_POST['password']);
	$email = $_POST['email'];
	if(empty($username) or empty($password)){
		echo "<p>Fields Empty</p>";
	}else {
		mysql_query("INSERT INTO users VALUES('', '$username', '$password', '2', 'a', '$email')");
	echo "</div><div id='box'><p>Successfully Registered</p></div>";
	}
}
?>


//- I want it to not insert multiple values with the same username, -//