unique selected value from dropdownlist

Question

i have a table Rubriqueintermediare with a rubriqueIntermediareId and rubriqueId as a foreign key from the table Rubrique:So in my rubrique intermediare i have a listbox of rubrique i want to compare viewdata list values with dropdownlist selected so to interdict the addition of the same rubriqueintermediare : i tried this code but it didn't work :

[HttpPost]
public JsonResult Save([DataSourceRequest] DataSourceRequest dsRequest, RubriquepointageVM vm)
{
    try
    {
        IEnumerable<RubriqueVM> listRubrique = RefDataManager.GetRefData<RubriqueVM>() as IEnumerable<RubriqueVM>;
        ViewData["CodeRubrique"] = new SelectList(RefDataManager.GetRefData<RubriqueVM>(), "RubriqueId", "CODELIBELLE");

        var Rubriques = listRubrique.AsQueryable();
         List<RubriquepointageVM> lstRubriqueinter = ServiceApplicatif.GetListe();

         if (!(listRubrique.Any(r => r.RubriqueId==(vm.RubriqueId)))){
             ModelState.AddModelError("RubriqueId", "ajout impossible ");
             return Json(ModelState.ToDataSourceResult());
         }else {
           RubriquepointageVM rub = ServiceApplicatif.Save(vm);
           return Json(new[] { vm }.ToDataSourceResult(dsRequest, ModelState));

           DataCache dataCache = new DataCache(CurrentSecurityContext.TenantID);
           dataCache.DropDataCache<RubriquepointageVM>();
            }

    }
    catch (Exception ex)
    {
        LoggerRubriquepointage.Error(string.Format("Exception : {0}", ex.Message.ToString()));
        throw new Exception("Erreur lors de l'enregistrement.");
    }
}

when i put a break point if i have added the same value from the listbox or not it excutes just the else:

 else {
       RubriquepointageVM rub = ServiceApplicatif.Save(vm);
       return Json(new[] { vm }.ToDataSourceResult(dsRequest, ModelState));

       DataCache dataCache = new DataCache(CurrentSecurityContext.TenantID);
       dataCache.DropDataCache<RubriquepointageVM>();
     }