As far as I know, we simply don't close questions as duplicates on Code Review. I think it's even been mentioned that a Code Review duplicate has to be basically an exact copy & paste of the previous question.
To some degree, this makes sense. Code Review questions and answers tend to be highly specific and tailored exactly to the user posting the question. It's hard for a question to be a duplicate on Code Review.
But how about iterative review questions? Can these sometimes be considered duplicates? If so, what sort of criteria do we have for iterative review questions being duplicates or non-duplicates? Is there a minimum amount of changes that need to be made to the source code? Is there a minimum amount of time that needs to elapse between asking? Should the previous question have an answer marked accepted?
I understand it's very unlikely a black & white issue, but I'm not certain that these are all necessarily non-duplicates. It's probably a gray area, but I think it'd be useful to discuss some guidelines on this topic.