ผมอยากจะทำโค้ดเกี่ยวกับห้าม insert ข้อมูลซ้ำ เมื่อ user มีข้อมูลอยู่ในบานข้อมูลเเล้ว

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script>
function back1(){
		alert("**ลงทะเบียนสำเร็จเเล้ว**กรุณาอย่ากรอกซ้ำ");
      location.href="check_status_student.php";
}
</script>
		
<?php
include('config.inc.php');

$_users_name=$_POST['users_name'];
$_users_surname=$_POST['users_surname'];
$_users_number=$_POST['users_number'];
$_users_number1=$_POST['users_sex'];
$_users_number1=$_POST['users_number1'];
$_users_faculty=$_POST['users_faculty'];
$_users_Major=$_POST['users_Major'];
$_users_tel=$_POST['users_tel'];
$_users_email=$_POST['users_email'];
$_users_dormitory=$_POST['users_dormitory'];



if($_POST['users_name']=="" || $_POST['users_surname']=="" || $_POST['users_number']==""||$_POST['users_sex']=="" || $_POST['users_number1']=="" || $_POST['users_faculty']=="" || $_POST['users_Major']=="" || $_POST['users_tel']=="" || $_POST['users_email']=="" || $_POST['users_dormitory']==""){
	
	echo "<script language='javascript'>alert('กรุณากรอกข้อมูลให้ครบ');history.back();</script>";	
}else {
	
	$strSQL = "SELECT * FROM detail_id_dorm where Detail_ID_name='".$_POST['users_dormitory']."'";
	$objQuery = mysql_query($strSQL);
	$nums=mysql_num_rows($objQuery);
	$objResult = mysql_fetch_array($objQuery);
	$limit=$objResult["limit_room"];
	$count=$objResult["count"];
	//echo "count === $count <br>";
	$total=$count+1;
	//echo "total === $total <br>";
	if($count>=$limit){
		echo "<script language='javascript'>alert('เต็มแล้ว !!!!!');history.back();</script>";
		}
	else {
		$SQL = "INSERT INTO register_dorm_online" ;
		$SQL .="(Study_name,Study_surname,Study_number,Study_sex,Study_number1, Study_faculty,Study_major,Study_tel,Study_e_mail,Study_dormitory,date) VALUES ('".$_POST["users_name"]."','".$_POST["users_surname"]."' ,'".$_POST["users_number"]."','".$_POST["users_sex"]."', '".$_POST["users_number1"]."','".$_POST["users_faculty"]."',
'".$_POST["users_Major"]."','".$_POST["users_tel"]."', '".$_POST["users_email"]."','".$_POST["users_dormitory"]."','".date("Y-m-d h:i:s")."')";
		$obj = mysql_query($SQL);
							if($obj){
								echo '<script>back1();</script>';
								}
							else{
								echo "<script language='javascript'>alert('error');history.back();</script>";
								}
		$strSQL = "UPDATE detail_id_dorm SET ";
		$strSQL .="count = '".$total."' ";
		$strSQL .="WHERE Detail_ID_name='".$_POST['users_dormitory']."' ";
		$objQuery=mysql_query($strSQL);
							if($objQuery){
								echo "<script language='javascript'>alert('save done');history.back();</script>";
								}
							else{
								echo "<script language='javascript'>alert('error');history.back();</script>";
								}
		}
	
}




/*
	


if($objQuery)
{
	echo "<script language='javascript'>alert('completed');history.back();</script>";
}
else
{
	echo "<script language='javascript'>alert('error');history.back();</script>";
}*/
mysql_close();


?>