[AllowAnonymous]
[HttpPost]
public ActionResult Login(LoginModel model, string returnUrl)
{
    if (ModelState.IsValid)
    {
        if (System.Web.Security.Membership.ValidateUser(model.UserName, model.Password))
        {
            FormsAuthentication.SetAuthCookie(model.UserName, model.RememberMe);
            if (Url.IsLocalUrl(returnUrl))
            {
                return Redirect(returnUrl);
            }
            else
            {
                return RedirectToAction("Index", "Home");
            }
        }
        else
        {
            ModelState.AddModelError("", "The user name or password provided is incorrect.");
        }
    }

    // If we got this far, something failed, redisplay form
    return View(model);
}

<authentication mode="Forms">
      <forms loginUrl="~/Account/Login" timeout="3600" />
    </authentication>

<!--<modules>
      <remove name="FormsAuthentication" />
</modules>-->

[AllowAnonymous]
[HttpPost]
public ActionResult Login(string userName, string password, string returnUrl)
{
if (ModelState.IsValid)
{
    if (HttpContext.User.Identity.IsAuthenticated)
    {
        return View(returnUrl);
    }
    else
    {
        if (System.Web.Security.Membership.ValidateUser(userName, password))
        {
            FormsAuthentication.SetAuthCookie(userName, false);
            if (Url.IsLocalUrl(returnUrl))
            {
                return RedirectToAction("Login", new { userName = userName, password = password, returnUrl = returnUrl });
                //return Redirect(returnUrl);
            }
            else
            {
                return RedirectToAction("Index", "Home");
            }
        }
    }
    else
    {
        ModelState.AddModelError("", "The user name or password provided is incorrect.");
    }
}

// If we got this far, something failed, redisplay form
return View(model);
}