prevent double upload using AsyncFileUpload

See more:

C#

ASP.NET

File-Upload

once i select the file using asyncfileupload, the file is already uploading to the server. once finished, i click the button, then the file originally uploaded upload again to the server.

i tried whatever i could, please advice :(

javascript

<script type="text/javascript">
        //upload file
        function uploadError(sender, args) {
            document.getElementById('lblStatusFupFile').innerText = args.get_fileName(), "" + args.get_errorMessage() + "";
        }

        function StartUpload(sender, args) {
            document.getElementById('lblStatusFupFile').innerText = 'Uploading...';
        }

        function UploadComplete(sender, args) {
            var filename = args.get_fileName();
            var contentType = args.get_contentType();
            var text = "File name: " + filename + " is uploaded.";
            document.getElementById('lblStatusFupFile').innerText = text;
        }       

</script>
HTML

<asp:ScriptManager ID="ScriptManager1" runat="server">
       </asp:ScriptManager>
       <asp:UpdatePanel ID="UpdatePanel1" runat="server">
           <Triggers>
               <asp:PostBackTrigger ControlID="btnUpload" />
           </Triggers>
       <ContentTemplate>
       <p>
           <asp:Button runat="server" Text="Upload" ValidationGroup="upload" ID="btnUpload" OnClick="btnUpload_Click"></asp:Button>
       </p>
       <p>
           <asp:AsyncFileUpload runat="server" OnClientUploadStarted="StartUpload" OnClientUploadComplete="UploadComplete" OnClientUploadError="uploadError" ThrobberID="ThrobberFile" UploadingBackColor="#66CCFF" Width="400px" FailedValidation="False" UploaderStyle="Modern" ID="fupFile" OnUploadedComplete="fupFile_UploadedComplete"></asp:AsyncFileUpload>
           <asp:Label ID="lblStatusFupFile" runat="server" Style="font-family: Arial; font-size: small; color: #000000;"></asp:Label>
           <asp:Label runat="server" ID="lblValAfuFile" Style="color: #FF0000; font-weight: 700"></asp:Label>
       </p>
           </ContentTemplate>
           </asp:UpdatePanel>

code behind:

 protected void fupFile_UploadedComplete(object sender, AjaxControlToolkit.AsyncFileUploadEventArgs e)
       {
               if (fupFile.HasFile)
               {
                   string strPath = MapPath("~/Uploads/files/") + Path.GetFileName(e.FileName);
                   fupFile.SaveAs(strPath);
               }
       }

protected void btnUpload_Click(object sender, EventArgs e)
       {
           if (this.fupFile.FileName == "")
           {
               this.lblValAfuFile.Text = "* File required.";
               return;
           }
     //some code
       }