为什么我的表单发送数据两次刷新后

我有一个表单在我的网站，但我不能解决一个问题。当我在表单框中写入一些文本时，它会将数据发送到数据库。当我点击刷新时，页面再次将相同的数据发送到数据库。我的代码有什么问题?

<?php
if(isset($_POST['submit']))
{   
$err = array();
$diss = $_POST['type'];
$sub = $_POST['sub'];
$msg = $_POST['msg'];
$uname = $_SESSION['uname'];
$date = "On " . date("F Y h:i:s A");
if (!isset($_SESSION['uname']))
$err[] = "You need to login";
else
{
if(empty($sub) && empty($msg))
$err[] = "All field required";
else
{
if(empty($sub))
$err[] = "Subject Requried";
if(empty($msg))
$err[] = "Message Requried";
 }
}
if(!empty($err))
{
foreach($err as $er)
{
echo "<font color=red><b>$er</b></font>";
}
}
else
{
$sql= mysql_query("INSERT INTO discussion VALUES ('', '$diss', '$sub', '$msg', '$uname', '$date' ) ");
if(!$sql)
echo "Can't submit your discussion";
else
{
echo "Discussion was submitted";
}
}   
}
?>
<form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post"     
name="discussion">
<table width="240" border="0" cellspacing="0" cellpadding="0">
<tr>
<td style="width:230;"><b>Select your Discussion</b>&nbsp;&nbsp;
<select name="type">
<?php   
$sqld = mysql_query("SELECT * FROM distype");
while($row = mysql_fetch_assoc($sqld))
{
$d_id = $row['d_id'];
$diss = $row['type'];
echo "<option value='$diss'>$diss</option>";
}
?>
</select></td>
</tr>
<tr>
<td><b>Subject</b></td>
</tr>
<tr>
<td><input type="text" name="sub" value="" size="33" class=""/></td>
</tr>
<tr>
<td><b>Message</b></td>
</tr>
<tr>
<td><textarea cols="30" rows="3" name="msg" class=""></textarea></td>
</tr>
<tr>
<td>&nbsp;</td>
</tr>
<tr>
<td><input type="submit" name="submit" value="Submit Form"><br>
<td></td>
</tr>
</table>