php ตรวจสอบค่าข้อมูลซ้ำ Save ข้อมูลไม่ได้ ทำไงดีครับ

<div class="col-md-5 col-md-offset-3">
            <p><font color="black"><h3><div class="animated bounceInUp"><i class="fa fa-database">&nbsp;สมัครสมาชิก</h3></font></i></p>
            <hr></hr>
             <form class="form-horizontal" name ="from1" method="post" action="saveregister.php">
                <div class="form-group">
                    <label for="inputUsername" class="col-sm-4 control-label">Username:</label>
                    <div class="col-sm-8">
                      <input name="username" type="text" class="form-control" placeholder="username">
                    </div>
                  </div>
                  <div class="form-group">
                    <label for="inputUsername" class="col-sm-4 control-label">Password:</label>
                    <div class="col-sm-8">
                      <input name="password" type="password" class="form-control" placeholder="Password">
                    </div>
                  </div>
                  <div class="form-group">
                    <label for="inputUsername" class="col-sm-4 control-label">ชื่อ-นามสกุล:</label>
                    <div class="col-sm-8">
                      <input name="name" type="text" class="form-control" placeholder="ชื่อ-นามสกุล">
                    </div>
                  </div>
                  <div class="form-group">
                  <div class="col-sm-3 col-sm-offset-4">
                  <input class="btn btn-default" type="submit" name="USER" id="USER" value="ส่งข้อมูล" />
                  </div>
                  <div class="col-sm-3">
                  <input class="btn btn-default" type="reset" name="#" id="#" value="ออกจากระบบ" onClick="window.location.href='logout.php'" />
                  </div>
                  </div>
       </form>

<?php
$objConnect = mysql_connect("localhost","root","1234") or die("Error Connect to Database");
$objDB = mysql_select_db("membernew");
mysql_query("SET NAMES UTF8");
$strSQL = "SELECT * FROM student WHERE username = '".$_POST["username"]."' ";
$objQuery = mysql_query($strSQL);
$objResult = mysql_fetch_array($objQuery);
if($objResult)
{
		exit ("<script> 
    alert ('username already exist.');
    window.location='register.php';
    </script>");
}
else
{

	$strSQL = "";
	$strSQL = "INSERT into student ";
	$strSQL .="(username,password,name1)";
	$strSQL .="('".$_POST["username"]."','".$_POST["password"]."','".$_POST["name"]."')";
	$objQuery = mysql_query($strSQL);
	if($objQuery)
	{
		echo "Save Done.";
	}
	else
	{
		echo "Error Save [".$strSQL."]";
	}
	}
mysql_close($objConnect);
?>