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.
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.
It certainly should be a certification that poster copied some keywords from the quote into the search box and pressed enter.
Use the search feature to make sure your quote has not already been posted.
Bad rule, as phrased.
If you are referring specifically to the literal meaning of 'sure' then fine. If you refer to the more casual meaning of "yeah, I...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.