This question is an exact duplicate of:
Now that we can retract flags, I think when a duplicate flag is retracted, the comment that was created as a result of the flag, should be removed as well.
This has a bit to do with this question:
It's a minor annoyance currently that the owner of said comment, needs to remove it. I don't think there's ever a case where someone would want to retract a duplicate flag, and not delete their comment as well.