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.net 4.0 webpage button click event firing twice
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        PostBackOptions optionsSubmit = new PostBackOptions(btnGo);
        btnGo.OnClientClick = "HideControlOnClick(this);";
        btnGo.OnClientClick += ClientScript.GetPostBackEventReference(optionsSubmit);
    }
}
protected void btnGo_Click(object sender, EventArgs e)
{
    bool locked = true;

    if (Session["ClickTime"] == null || (DateTime)Session["ClickTime"] < DateTime.Now.AddSeconds(-20))
    {
        Session["ClickTime"] = DateTime.Now;
        locked = false;
    }

    WriteToLog(1, locked);

    if (Page.IsValid && !locked)
    {
        locked = true;

        WriteToLog(2, locked);

        // Do all my processing
    }

    WriteToLog(3, locked);
}

<script language="javascript" type="text/javascript">

    function HideControlOnClick(btnGo)
    {
        // IE uses className for the css property.
        btnGo.setAttribute('className', 'hide');

        document.getElementById('MainContent_imgWait').setAttribute('className', 'show');

        setTimeout("UpdateImg('MainContent_imgWait','Images/loading.gif');",50);
    }
    function UpdateImg(ctrl, imgsrc)
    {
        var img = document.getElementById(ctrl);
        img.src = imgsrc;
    }
</script>

btnGo.OnClientClick = "return HideControlOnClick(this);";


function HideControlOnClick(btnGo)
{
   if (btnGo["My_Is_Clicked"]) {
      // already clicked, ignore
      return false;
   }
   btnGo["My_Is_Clicked"] = true;
   ...
   return true;
}