Show only one UpdateDialog at a time

PamhagenSysCtrl/App.xaml.cs

@@ -67,6 +67,9 @@ namespace PamhagenSysCtrl {
 
         public static async Task CheckForUpdates(bool showResult = false) {
             if (Config.UpdateUrl == null) return;
+            foreach (Window w in Current.Windows) {
+                if (w is UpdateDialog) return;
+            }
 
             try {
                 var latest = await UpdateService.GetLatestInstallerUrl(Config.UpdateUrl);