Prevent adding new labels if there are some to add

lib/pages/list/task_edit.dart

@@ -379,6 +379,11 @@ class _TaskEditPageState extends State<TaskEditPage> {
   }
 
   _createAndAddLabel(String labelTitle) {
+    // Only add a label if there are none to add
+    if(labelTitle == '' || _suggestedLabels.length > 0) {
+      return;
+    }
+
     Label newLabel = Label(title: labelTitle);
     VikunjaGlobal.of(context)
         .labelService