title: Xamarin Forms MessagingCenter Subscribe called two times
body: <p>I'm clicking on a product item in listview in product page viewmodel to show a popup(using rg.plugin popup) for selecting one of the product variants.After selecting variant,i am sending the selected variant to product page using messagingcenter from variant popup page viewmodel,subscribed in product page viewmodel constructor. working fine there.when i navigate to the previous page and then came back to this product page for adding one or more variant to the 
  same previously selected product,Messagingcenter subscribe called twice and product value increased twice.Tried to subscribe in the product page onappearing and unsubscribe in disappearing method.still calling two times? How to solve this issue?</p>
  
  <p>calling popup:</p>
  
  <pre><code>               var result = await dataService.Get_product_variant(store_id, product_id);
                  if (result.status == "success")
                  {
                      ind_vis = false;
                      OnPropertyChanged("ind_vis");
  
                      App.Current.Properties["product_variant_result"] = result;
                      App.Current.Properties["cartitems"] = purchaselist;
                      App.Current.Properties["selected_product"] = product_List2 ;
                      await PopupNavigation.Instance.PushAsync(new Popup_variant());                   
                  }
  </code></pre>
  
  <p>popup viewmodel: sending message</p>
  
  <pre><code>        public Popup_variant_vm()
          {
              Radio_btn = new Command&lt;Product_variant_list2&gt;(Radio_stk_tapped);
              product_variant_list = new List&lt;Product_variant_list2&gt;();
              purchaselist = new ObservableCollection&lt;Product_list2&gt;();                       
              show_variants();            
          }
  
          internal void Confirm_variant()
          {
              if(App.Current.Properties.ContainsKey("selected_variant"))
              {
                   MessagingCenter.Send&lt;Popup_variant_vm, object&gt;(this, "selected_variant", App.Current.Properties["selected_variant"]); //Message send from popup to product page                    
              }
              else
              {
                  DependencyService.Get&lt;IToast&gt;().LongAlert("Please select any size");
              }
          }
  </code></pre>
  
  <p>product page viewmodel: subscribed here..called twice when navigating from previous page to this                  </p>
  
  <pre><code>    public Store_page()
      {
         InitializeComponent();
      }
      protected override void OnAppearing()
      {
          base.OnAppearing();
  
          var vm = new store_page_vm();
          vm.Navigation = Navigation;
          BindingContext = vm;
  
          MessagingCenter.Unsubscribe&lt;Popup_variant_vm, object&gt;(this, "selected_variant");
          MessagingCenter.Subscribe&lt;Popup_variant_vm, object&gt;(this, "selected_variant",async (sender, selected_variant) =&gt;
          { 
              var vm1 = BindingContext as store_page_vm;
              vm1?.Addcart2(selected_variant);// called twice
          });            
      } 
  </code></pre>
  
  <p>unsubscribed in product cs page</p>
  
  <pre><code>protected override void OnDisappearing()
          {
              var vm = BindingContext as store_page_vm;
              vm?.Save_cart();    
              MessagingCenter.Unsubscribe&lt;Popup_variant_vm&gt;(this, "selected_variant");    
          }
  </code></pre>