สอบถามการเชคค่าก่อนที่จะ insert ข้อมูลลงMySQL อยากให้เช็คข้อมูลซ้ำก่อนครับ

<?
$objConnect = mysql_connect("localhost","root","root") or die("Error Connect to Database");
$objDB = mysql_select_db("mydatabase");
$strSQL = "SELECT * FROM customer WHERE CustomerID = '".$_POST["txtCustomerID"]."' ";
$objQuery = mysql_query($strSQL);
$objResult = mysql_fetch_array($objQuery);
if($objResult)
{
		echo "CustomerID already exist.";
}
else
{

	$strSQL = "";
	$strSQL = "INSERT INTO customer ";
	$strSQL .="(CustomerID,Name,Email,CountryCode,Budget,Used) ";
	$strSQL .="VALUES ";
	$strSQL .="('".$_POST["txtCustomerID"]."','".$_POST["txtName"]."','".$_POST["txtEmail"]."' ";
	$strSQL .=",'".$_POST["txtCountryCode"]."','".$_POST["txtBudget"]."','".$_POST["txtUsed"]."') ";
	$objQuery = mysql_query($strSQL);
	if($objQuery)
	{
		echo "Save Done.";
	}
	else
	{
		echo "Error Save [".$strSQL."]";
	}
	}
mysql_close($objConnect);
?>

$sql = "select * from absence where employee_id='".$employee_id."' and (stdate between '$startDate' and '$endDate') and (endate between '$startDate' and '$endDate')";
$res = mysql_query($sql) or die(mysql_error());
if(mysql_num_rows($res)){
   $data = mysql_fetch_assoc($res);
   echo "พบเอกสารเลขที่ '{$data["absence_no"]}' / '{$data["document_subject"]}' ในฐานข้อมูลครับ!<br />\n ";
   echo "ไม่สามารถบันทึกทำการลาครั้งนี้ได้ครับ";
}else{
   $RsUpseq = mysql_query("UPDATE tab_sequence SET document_seq = document_seq + 1");
   $RsUpseq = Null;
   $Resseq = mysql_query("SELECT document_seq AS docseq FROM tab_sequence");
   $result = mysql_fetch_array($Resseq);
   $docno = $result["docseq"];
   $strSQL = " INSERT INTO absence " ; 
   $strSQL .= "( absence_no,employee_id,document_subject,type_no,";   
   $strSQL .= " stdate, endate, status , dayno , entrancedate ) ";
   $strSQL .= " VALUES('$doc_no' , '$employee_id' , '$docsubj', ";
   $strSQL .= " '$txt_doc_title' , '$startDate' , '$endDate', ";
   $strSQL .= " 'W', $sumDay , '$entrance_date')";
   $objQuery = mysql_query($strSQL) or die(mysql_error());
   if(mysql_affected_rows()){
		echo "Save Done.";
    }
}