Disable Button After First Click to Prevent the Second Click

Javascript

<script language="javascript" type="text/javascript">
    function disableButton(sender,group)
    {
        Page_ClientValidate(group);
        if (Page_IsValid)
        {
            sender.disabled = "disabled";
            __doPostBack(sender.name, '');
        }
    }
</script>

HTML

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
      <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1" ValidationGroup="i"
          ErrorMessage="RequiredFieldValidator">*</asp:RequiredFieldValidator>&nbsp;<br />
    <asp:Button runat="server" ID="btnSave" Text="Save" OnClick="Save" OnClientClick="disableButton(this,'i')" UseSubmitBehavior="false" ValidationGroup="i" />

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