Ooops. To redeem my tarnished honor, I propose an algorithmic solution to the duplicate quote problem: a full list of quotes indexed by author (of the quote). Checking to see if a quote has already been posted would then be a fast operation.
The hard part would then be making that list algorithmically. An easier algorithmic method would be to do approximate string matches with previous quote threads, using something like the Smith-Waterman algorithm for pairwise local sequence alignment. This is what biologists do when they have a gene sequence and want to know if something like it is already in the databases, and there's no reason why the method shouldn't also apply just as well to English text.
The way this would look to users is just a text box where you paste in the quote, and it'll tell you if the quote has been posted before. Even easier to use than a full list of quotes.
Every month on the month, Less Wrong has a thread where we post Deep Wisdom from the Masters. I saw that nobody did this yet for December for some reason, so I figured I could do it myself.
* Please post all quotes separately, so that they can be voted up/down separately. (If they are strongly related, reply to your own comments. If strongly ordered, then go ahead and post them together.)
* "Do not quote yourself." --Tiiba
* Do not quote comments/posts on LW/OB. That's like shooting fish in a barrel. :)
* No more than 5 quotes per person per monthly thread, please.