THIS ADDON RESOLVES THE BLIZZARD BUG FOR DUPLICATE MESSAGES ON GENERAL ETC.
1.3.0 also filters your own for this

This package was created for personal use, as well as friends and family to cut down on duplicate messages, gold seller spam, anal spam, and help in finding raid groups by highlighting messages you're looking for. It does the job without any fuss or massive download, so don't expect any fancy GUIs.

What's in it -

• Repeat message filter. You never see people's repeated spam on General, Trade, Local Defense. Also solves Blizzard's double-post bug.
  
  • max size 50 messages, 999 seconds. Default is 25messages:120second no-repeat across channels 1-4.
    
• Gold spam monitoring and blocking of author for rest of session. Notification optional (off by default, so it silently hides messages). No autoreport currently.
  
  • Currently fixed list of known sellers for Chamber of Aspects Europe.
    
  • Feel free to edit the list in the code for your own use if you wish.
    
• 'Anal' spam blocking and counter. Notification optional (off by default, so it silently hides messages).
  
• Highlighter (scan function) - set a keyword to look for, eg ICC10, and AG will make the messages red which contain 'ICC10' or 'ICC 10' (all non-alphanumeric characters are stripped out).

It doesn't block everything, but it cuts a good chunk of it out, and it doesn't use massive amounts of resource doing it.

use /ag to show the settings.