The room list behavior that we see  in Outlook is by design. When we  use a Room List  for a meeting, it is stored in the  Most Recently Used entries in the registry. When we create a new meeting, we will see this MRU entry in the top of the Room Lists . The same Room List will be seen again in the drop down which is accessed from the Exchange Server/ Online.

To prevent the duplicate entries seen in the Room List, create the below registry entry with blank data to disable the  Most Recently Used  Room List in Outlook.

Path: HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Preferences

Key: RoomFinderRecentRooms
Key: RoomFinderRecentRoomList

If these entry already exist just empty the values.

After Outlook Restart:

 Only single instance of the rooms list now showing!