โค้ด php mysql เพื่อบล๊อกค่าที่ซ้ำกัน ในขณะ insert ผมต้องทำยังไงครับ

<?
$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);
?>