Disable the button after the first click and submit the form in asp.net.

DisableAspButton.trick

<asp:Button ID="btnSubmit"
  ValidationGroup="confirmOrder"
  OnClientClick="submitOrder()"
  OnClick="SubmitOrder"
  Text="Pay Now" 
  CssClass="inputSubmit" runat="server"
  UseSubmitBehavior="false"
/>
<script type="text/script"> 
  function submitOrder() {
      var isValid = false;
      isValid = Page_ClientValidate('confirmOrder');
      if(isValid) {
          document.getElementById('btnSubmit').disabled = true;
      }
      return isValid;
  }
</script>