fix: #8127 user join system message duplicated

- Now showing user-join message for chat owner in addition to the newly
  added user
- If a modal already exists for a room when createModal is called, don't
  return null, return the modal

public/src/modules/chat.js

@@ -145,7 +145,7 @@ define('chat', [
 		require(['scrollStop', 'forum/chats', 'forum/chats/messages'], function (scrollStop, Chats, ChatsMessages) {
 			app.parseAndTranslate('chat', data, function (chatModal) {
 				if (module.modalExists(data.roomId)) {
-					return callback(null);
+					return callback(module.getModal(data.roomId));
 				}
 				var uuid = utils.generateUUID();
 				var dragged = false;

src/messaging/rooms.js

@@ -53,6 +53,7 @@ module.exports = function (Messaging) {
 			db.sortedSetAdd('chat:room:' + roomId + ':uids', now, uid),
 		]);
 		await Promise.all([
+			Messaging.addSystemMessage('user-join', uid, roomId),	// chat owner should also get the user-join system message
 			Messaging.addUsersToRoom(uid, toUids, roomId),
 			Messaging.addRoomToUsers(roomId, [uid].concat(toUids), now),
 		]);