WebApi Action filter called twice

Question

My WebApi filter method OnActionExecuted is being called twice. My filter (I make it as simple as possible):

   public class NHibernateActionFilter : ActionFilterAttribute
   { 
        //  [Inject]
        //   public ISessionFactoryProvider sessionFactoryProvider { get; set; }
        public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
        {
           var a = 5;
           var b = a;
           //new BaseSessionProvider(sessionFactoryProvider).EndContextSession();
        }
    }

My setup:

    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();
        WebApiConfig.Register(GlobalConfiguration.Configuration);
        //http://stackoverflow.com/questions/9521040/how-to-add-global-asp-net-web-api-filters
        FilterConfig.RegisterWebApiFilters(GlobalConfiguration.Configuration.Filters);
    }

    public class FilterConfig
    {

        public static void RegisterWebApiFilters(System.Web.Http.Filters.HttpFilterCollection filters)
        {
             filters.Add(new NHibernateActionFilter());
        }
     }

In debugger I catch OnActionExecuted twice with the same actionExecutedContext. Why?

UPD

Controller
public class BankSmsController : ApiController
{
         [AcceptVerbs(HttpVerbs.Get)]
         public int GetTest()
         {
             return 1;
         }
}

Answer 1

I have a suspicion, that this strange behavior can be fixed by either overriding AllowMultiple property of filter and returning false, or applying AttributeUsage attribute with AllowMultiple set to false too (this influences on default implementation of AllowMultiple property of filter.

At least in our project this helped (we have filters injected via Autofac).