Mailform that sends mail every refresh

[DKY]

I have a simple mailform that does one of these

<FORM ACTION="<?$_SERVER['PHP_SELF']?>" METHOD="POST" NAME="PO_History">
<p align=center> 
<table width="550" cellspacing="0" cellpadding="0" border="0" bordercolor="#000000"> 
<tr> 
<td><p align=center><font id='header'>P.O. History</font><br><br></td> 
</tr> 
<tr> 
<td><p align=center><TEXTAREA NAME="po_history" ROWS="20" COLS="100%" WRAP></TEXTAREA></td> 
<tr> 
<td><br><p align=center><INPUT TYPE="submit" NAME="submit" VALUE="Submit P.O. History"></td> 
</tr> 
</table> 


<? 


if (isset($_POST['submit']) && ($_POST["submit"] == "Submit P.O. History"))
{ 
$Mail_Success=mail('[e-mail]','INCOMING HOTLINE','http://mysite/hotlines/PDC_Hotline.php');
}  

$disconnectmysql = mysql_close($connectmysql);

?> 
</form> 

and it even runs the script every time I refresh. I'm thinking all I have to do is his the submit button once and it never clears itself out or something so it thinks its forever submitted. I don't know, any advice is more than welcome. TIA