Now, I raised "duplicate" flag. And I don't argue with the fact that "another reason" should be disabled. But the text should be different, as I didn't raise any "another reason" flags on this question.
The text under the second option shown is untrue, but it may be OK to leave both messages identical. Just make them:
you have already raised a flag from the same moderation queue
Of course my wording may be less than perfect, it's the general idea. That way it will be true without any changes in code.