ทำไมเวลาเพิ่มข้อมูลลงฐานข้อมูลแล้วกด refresh หน้าเพจ กลายเป็นบันทึกข้อมูลซ้ำกัน 2 รอบ

<?php
			//*** Add Condition ***//
if($_POST["cmd"] == "add")
{
	$sqlMax="SELECT MAX(tit_id) FROM title ";
	$result=mysql_query($sqlMax);
	$rs=mysql_fetch_array($result);
	$id_max=$rs[0]+1;
	$strSQL = "INSERT INTO title ";
	$strSQL .="(tit_id,tit_name) ";
	$strSQL .="VALUES ";
	$strSQL .="('$id_max','".$_POST["titlename"]."') ";
	$objQuery = mysql_query($strSQL);
	if(!$objQuery)
	{
		echo "Error Save [".mysql_error()."]";
	}else{
		echo"<script>alert('บันทึกข้อมูลเรียบร้อย');</script>";
			}
}

//*** Update Condition ***//
if($_POST["cmd"] == "update")
{
	$strSQL = "UPDATE title SET ";
	$strSQL .="tit_id = '".$_POST["titid"]."' ";
	$strSQL .=",tit_name = '".$_POST["titname"]."' ";
	$strSQL .="WHERE tit_id = '".$_POST["titid"]."' ";
	$objQuery = mysql_query($strSQL);
	if(!$objQuery)
	{
		echo "Error Update [".mysql_error()."]";
	}
	
}

//*** Delete Condition ***//
if($_GET["Action"] == "Del")
{
	$strSQL = "DELETE FROM title ";
	$strSQL .="WHERE tit_id = '".$_GET["CusID"]."' ";
	$objQuery = mysql_query($strSQL);
	if(!$objQuery)
	{
		echo "Error Delete [".mysql_error()."]";
	}
	
}
	$sqlMax="SELECT MAX(tit_id) FROM title ";
	$result=mysql_query($sqlMax);
	$rs=mysql_fetch_array($result);
	$id_max=$rs[0]+1;
		?>
	<script language="javascript">
		function check()
		{
			if(document.frmmain.titlename.value == "")
			{
				alert('กรุณากรอกคำนำหน้านาม');
				document.frmmain.titlename.focus();
				return false;
			}	
		}
	</script>
     <table width="100%" border="0" align="center" cellpadding="1" cellspacing="1">
		
		<form name="frmmain" method="post" onSubmit="JavaScript:return check();" action="<?=$_SERVER["PHP_SELF"];?>">
		<input type="hidden" name="cmd" value="">
		<table width="89%" border="1" align="center" cellpadding="0" cellspacing="0" bordercolor="#E9E9E6">
		<tr> 
		<td></br>
		<table width="500" border="1" align="center" cellpadding="0" cellspacing="0">
		<tr>
		<td colspan="2" bgcolor="#D7EBFF" height="25"><div align="center" height="10">จัดการข้อมูลคำนำหน้านาม</div></td>
		</tr>
		<!--<tr>
		<td><div align="right">รหัสคำนำหน้านาม</div></td>
		<td>&nbsp;&nbsp;<? echo $id_max?></td>
		</tr>-->
		<tr>
		<td width="150"><div align="center">คำนำหน้านาม&nbsp;</div></td>
		<td width="350"><input type="text" name="titlename" id="titlename" size="50"></td>
		</tr>
		<tr>
		<td colspan="2" align="center">
		<input type="submit" name="submit" id="submit" value="บันทึก" onClick="frmmain.cmd.value='add';frmmain.submit();"/>
		<input type="reset" name="clear" value="ยกเลิก"></td>
		</tr>
		</table>
		</br>
		<?php
			$sql ="SELECT * FROM title ORDER BY tit_name ASC";
			$rs = mysql_query($sql);
			$num = mysql_num_rows($rs);
			echo mysql_error();
		?>
		

		<table width="500" border="1" align="center" cellpadding="2" cellspacing="0">
		<tr bgcolor="0099ff">
		<td width="70" valign=top><div align="center" >ลำดับที่</div></td>
		<td width="200" valign=top><div align="center" >คำนำหน้านาม</div></td>
		<td width="60" valign=top><div align="center" >แก้ไข</div></td>
		<td width="60" valign=top><div align="center" >ลบ</div></td>
		</tr>
		<?php
		$i=1;
		while($row = mysql_fetch_array($rs)){
		?>

		<?
			if($row["tit_id"] == $_GET["CusID"] and $_GET["Action"] == "Edit")
			{
		?>
		<tr bgcolor="ffffcc">
		<td valign=top><div align="center" ><? echo $i?>
		<input type="hidden" name="titid" value="<?=$row["tit_id"];?>">
		</div></td>
		<td valign=top><div align="center" ><input type="text" name="titname" value="<?=$row["tit_name"];?>"></div></td>
		<td valign=top><div align="center" >
		<input name="submit" type="submit" id="submit" value="แก้ไข" OnClick="frmmain.cmd.value='update';frmmain.submit();">
		</td>
		<td valign=top><div align="center" > <input name="btnAdd" type="button" id="btnCancel" value="ยกเลิก" OnClick="window.location='<?=$_SERVER["PHP_SELF"];?>';"></div></td>
		</tr>
		<?
			}
		else
		{
		?>
		<tr bgcolor="ffffcc">
		<td valign=top><div align="center" ><? echo $i?></div></td>
		<td valign=top><div align="center" ><? echo $row['tit_name'];?></div></td>
		<td valign=top><div align="center" >
		<a href="<?=$_SERVER["PHP_SELF"];?>?Action=Edit&CusID=<?=$row["tit_id"];?>"><img
			src="/ot/images/edit.png" width="20" height="20" border="0"
			align="middle" /></a>
		<td valign=top><div align="center" > <a href="JavaScript:if(confirm('ยืนยันการลบข้อมูล <? echo "&nbsp;".$row['tit_name'];?>')==true)
			{window.location='<?=$_SERVER["PHP_SELF"];?>?Action=Del&CusID=<?=$row["tit_id"];?>';}"><img
			src="/ot/images/delete.png" width="20" height="20" border="0"
			align="middle" /></a></div></td>
		</tr>
		<?}
			$i++; 
			}?>
		
		</form>
		
		</td>
		</tr>
		</table>
		</br>