Prevent users from submitting forms twice in Sitecore's Web Forms for Marketers.

gistfile1.cs

using System;
using Sitecore.Form.Core.Renderings;
using Sitecore.Form.Web.UI.Controls;

namespace Custom.Form.Web.UI.Controls
{
    public class PreventDoubleClickFormRender : FormRender
    {
        // disables submit button if group validator for submit is valid
        private const string PreventDoubleSubmitJs = @"function disableSubmitButton(groupValidator, submitButton) {{ $(submitButton).disabled = Page_ClientValidate(groupValidator);}}";

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            Page.ClientScript.RegisterClientScriptBlock(GetType(), ID, PreventDoubleSubmitJs);

            var formSubmitId = FormInstance.ID + SitecoreSimpleForm.prefixSubmitID;
            var submit = FormInstance.FindControl(formSubmitId) as SubmitButton;
            if (submit != null)
            {
                submit.Attributes["onclick"] += string.Format("disableSubmitButton('{0}', '{1}');", submit.ValidationGroup, submit.ClientID);
            }
        }
    }
}