การตรวจสอบว่าไอดีซ้ำกันหรือไม่ก่อนที่จะเข้าสู่หน้ากรอกข้อมูลนั่นมีรูปแบบอย่างไรครับ

<?php
$projectid=$_REQUEST["projectid"];
$applicantidno=$_REQUEST["applicantidno"];

if(!is_numeric($projectid)) header("Location:register_project.php");
require_once("inc/project.inc.php");
$ProjectFunction = new Project;

$Result = $ProjectFunction->GetProject($projectid);
if(!count($Result)) header("Location:register_project.php");
$ProjectName = $Result[0]["projectname"];
?>

function CheckApplicantNo(){
ObjName = 'applicantidno';
	f = document.forms[0];
	var xmlHttp = AjaxCheck();
	xmlHttp.onreadystatechange=function()
	{
		if(xmlHttp.readyState==4){
			Val = xmlHttp.responseText;
			if(Val=="0") f.submit();//return true;//New Applicant No
			else if(Val=="404"){
				document.getElementById("DspError").innerHTML=f[ObjName].value+" Invalid project. <br> "+f[ObjName].value+" หลักสูตรไม่ถูกต้อง";
				f[ObjName].value = "";
				f['Submit'].disabled = false;
				f[ObjName].focus();
			}else{//Druplicate Applicant No
				document.getElementById("DspError").innerHTML="Applicant No. "+f[ObjName].value+" is already used.<br>ใบสมัครเลขที่ "+f[ObjName].value+" ถูกใช้แล้ว";
				f[ObjName].value = "";
				f['Submit'].disabled = false;
				f[ObjName].focus();
			}//end if
		}//end if
	}//End function
	pmeters = "projectid=<?php echo $projectid;?>";
	pmeters += "&"+ObjName+"="+f[ObjName].value+"&projectid=<?php echo $projectid;?>";

	theURL = "checkapplicantno.php";
	xmlHttp.open("POST",theURL,true);
	xmlHttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
	xmlHttp.setRequestHeader("Content-length", pmeters.length);
	xmlHttp.setRequestHeader("Connection", "close");
	xmlHttp.send(pmeters);

}//End CheckApplicantNo

function EnableSubmit(f,ObjVal){
  f['Submit'].disabled= (ObjVal.length>=10) ? false:true;
}//End EnableSubmit


function goSubmit(){
	f = document.forms[0];
	return ((f['applicantidno'].value.lenth>=10) && (!f['Submit'].disabled))? true:false;
}//End goSubmit