The flag queue lists automatically-detected duplicate answers.
According to Duplicate answers (not questions) and other questions on MSO, there is nothing wrong with such duplicate answers—except insofar as they are potentially indicative of a duplicate question.
So, having verified that the answers do not merit a flag for any other reason (and having flagged the questions as duplicates if appropriate: although if that was the purpose of this exercise, shouldn't the system have raised automatic flags on the questions instead?), how should one respond to these flags?
It seems that the only option is to flag for moderator attention, either:
because "I do not agree with the existing flags on this answer, they are incorrect and should be ignored." (invalid flag), even though I do agree that the answers are indeed duplicates; or
for a custom reason.
Neither of the above really feel very appropriate. Perhaps if the flag was instead "suspected spam" (or similar) it would be easier to flag as being invalid?