I have a form in my website, but I can't fix one problem. When I write some text in the form box, it sends the data to the database. When I hit refresh, the page sends the same data again, to the database. What is the problem with my code?

<?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>  
<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> </td>
</tr>
<tr>
<td><input type="submit" name="submit" value="Submit Form"><br>
<td></td>
</tr>
</table>

On successful form submit you need to reload the url or redirect him somewhere to prevent user from inserting data to the database.

$sql= mysql_query("INSERT INTO discussion VALUES ('', '$diss', '$sub', '$msg', '$uname', '$date' ) ");
if(!$sql)
echo "Can't submit your discussion";
else
{
   header("Location: page.php?mode=success");
   //or
   header("Location: ".$_SERVER['REQUEST_URI']); //which will just reload the page
}