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 checked this with search" then I disagree and would suggest that you implement the "it's not worth it for you" contingency.
I've always found the search engine quite clunky, and of questionable reliability. I think an actually explicit social norm will solve most of the problem. That said, I won't be put out if posting rationality quotes is not worth my effort.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.