I am developing Umbraco 7 MVC application and my requirement is to add Item inside Umbraco. Item name should be unique. For that used the below code but I am getting the error "Oops: this document is published but is not in the cache (internal error)"

      protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplication,
            ApplicationContext applicationContext)
        {
            ContentService.Publishing += ContentService_Publishing;
        }


        private void ContentService_Publishing(IPublishingStrategy sender, PublishEventArgs<IContent> e)
        {
            try
            {
               if(newsItemExists)
               {
                  e.Cancel = true;
               }
            }
            catch (Exception ex)
            {
                e.Cancel = true;
                Logger.Error(ex.ToString());
            }
        }

Then I tried adding code to unpublish but its not working i.e the node is getting published. Below is my code

private void ContentService_Publishing(IPublishingStrategy sender, PublishEventArgs<IContent> e)
        {
            try
            {
               int itemId=1234; //CurrentPublishedNodeId
               if(newsItemExists)
               {
                  IContent content = ContentService.GetById(itemId);
                  ContentService.UnPublish(content);
                  library.UpdateDocumentCache(item.Id);
               }
            }
            catch (Exception ex)
            {
                e.Cancel = true;
                Logger.Error(ex.ToString());
            }
        }

But with the above code, if you give the CurrentPublishedNodeId=2345 //someOthernodeId its unpublished correctly.

Can you please help me on this issue.