It's already considered bad form to repeat a quote. I thought this was one of the listed rules, but since it isn't (at least in the current thread) I agree that it should be added.
No repeats should be in the rules, but a posting on the rationality quotes pages is not and should not be a certification that the posters has investigated and is confident that there is no repeat.
If I had to investigate that hard before posting on that thread, I'd never do it because it wouldn't be worth the investment of time. And the real consequences for repeating a rule are so low. In short:
Avoid repeating quotes.
Good rule.
Use the search feature to make sure your quote has not already been posted.
Bad rule, as phrased.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.