Don't add the same message twice (and add a comment so I don't make this mistake a third time)

Author: Trolldemorted

Signal-Windows/Controls/VirtualizedMessagesCollection.cs

@@ -147,7 +147,7 @@ public int Add(object value, bool hideUnreadMarker)
                 LoadPage(pageIndex);
             }
             var cacheLine = MessageStorage[pageIndex];
-            cacheLine.Add(message);
+            cacheLine.Insert(inpageIndex, message); // If the page was loaded by LoadPage before, it already contains our message
             int virtualIndex = GetVirtualIndex(inConversationIndex);
             Logger.LogTrace("Add() Index={0} VirtualIndex={1}", inConversationIndex, virtualIndex);
             CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, message, virtualIndex));