cara mengatasi double submit ketika refress

mohon bantuannya gimna cara mengatasi double imput data ketika internet lemot dan refress dah nyoba cara di google hasilnya error teruss form

 <div class="form-group">
   <label class="col-md-3 col-xs-12 control-label">SAP CODE :</label>
   <div class="col-md-6 col-xs-12">
   <div class="input-group">
   <span class="input-group-addon"><span class="fa fa-pencil"></span></span>
<input name="stockno" type="text" onkeyup='autoComplete();' id="inputan" class="form-control" placeholder="......" required><div id='hasil'></div>
                                            </div>
                                        </div>
                                    </div>


  <div class="form-group">
  <label class="col-md-3 col-xs-12 control-label">DESCRIPTION :</label>
  <div class="col-md-6 col-xs-12">

  <div class="input-group">
  <span class="input-group-addon"><span class="fa fa-pencil"></span></span>
<input type="text"  class="form-control" id="item_name" placeholder="......" required>
                                            </div>
                                        </div>
                                    </div>


                                      <div class="form-group">
  <label class="col-md-3 col-xs-12 control-label">LOCATION :</label>
  <div class="col-md-6 col-xs-12">

  <div class="input-group">
  <span class="input-group-addon"><span class="fa fa-pencil"></span></span>
<input type="text"  class="form-control" id="location" placeholder="......" required>
                                            </div>
                                        </div>
                                    </div>


                                      <div class="form-group">
  <label class="col-md-3 col-xs-12 control-label">STOCK :</label>
  <div class="col-md-6 col-xs-12">

  <div class="input-group">
  <span class="input-group-addon"><span class="fa fa-pencil"></span></span>
<input type="text"  class="form-control" id="sap" placeholder="......" required>
                                            </div>
                                        </div>
                                    </div>


 <div class="form-group">
  <label class="col-md-3 col-xs-12 control-label">UNIT :</label>
  <div class="col-md-6 col-xs-12">

  <div class="input-group">
  <span class="input-group-addon"><span class="fa fa-pencil"></span></span>
<input type="text"  class="form-control" id="unit" placeholder="......" required>
                                            </div>
                                        </div>
                                    </div>



<div class="form-group">
<label class="col-md-3 col-xs-12 control-label">QTY ACT :</label>
<div class="col-md-6 col-xs-12">

<div class="input-group">
<span class="input-group-addon"><span class="fa fa-pencil"></span></span>
<input type="text" name="act" class="form-control"  placeholder="......" required>
                                            </div>
                                        </div>
                                    </div>



<div class="panel-footer">

<input type="submit" name='submit' class="btn btn-primary pull-right" value="ADD STOCK" >

                                        </div>
                                      </div>
                                    </div>
                                  </form>
                    <!-- END CONTENT FRAME BODY -->

prosess

<?php

    include_once '../connection.php';    //memanggil data base
if(isset($_GET['stockno']))             //memanggil menggunakan stockno
{
$stockno= $_GET['stockno'];

$act = $_GET['act'];
$qq = $act;


}

                                    // Set path folder tempat menyimpan fotonya

if(isset($_GET['submit']))
{
$stockno = $_GET['stockno'];

                             //memanggil isi data kolom sap and act berdasarkan stockno di tb stock_in

$result1 = mysql_query("SELECT * from stock_in where stockno = ".$stockno." ");
while($row = mysql_fetch_array($result1))
{
$update=$row['act'];




}

}

                                 //menjumlahkan nilai qty yg ada dengan qty yang baru

$sql = mysql_query("UPDATE  stock_in SET act=($qq + $update) WHERE stockno='$stockno'");

                                //alrm jika data sudah masuk

        $retval = mysql_query( $sql);
              echo'<script>';
              echo'alert("DONE...!");';
              echo'window.location.href="sp_inv.php";';
              echo'</script>';



              $result3 = mysql_query("SELECT * from stock_in where stockno = ".$stockno." ");
while($row = mysql_fetch_array($result3))
{
$ac=$row['act'];
$sa=$row['sap'];
$deff=$ac - $sa;


}

mysql_query("UPDATE  stock_in SET deff=$deff WHERE stockno='$stockno'");


?>