Remove duplicates from Organizer catalogue

In theory, you should not have any true duplicates.  But the simple answer to your question is that you need to (find and) select the duplicates (using Shift-Click and/or Ctrl-click (on Windows) and press the Delete key.  You will then be given an option whether you also want to delete the selected files from your hard drive.  If this doesn't answer your question, we need more details.