Why not put the same logic into a plugin for your chat client and have it warn you if you are about to send a duplicate phrase. Obviously it wouldn't catch everything ever said if you weren't always online, but if you were on a lot it would at least catch some cases, allowing you to avoid punishment.

Of course, this probably spirals out of control quickly. I'm surprised no one has joined with a war script tailored specifically to this type of moderator. Just give a bot a large corpus of blog entries, have it join late at night when no one is awake, then say every sentence it has in the channel... the table of things-that-have-been-said would fill up with all these bogus phrases. Might need a while to be effective, but it would be a fun experiment in moderator bombing.