Often times I see on a question a comment like this:
If I feel that is a duplicate, should I also add a flag? Or should I not, since there's already (probably) a flag on the question? Or, should I just upvote the comment to show support for it?
In cases like this, upvote the comment (if it really has merit, because sometimes it doesn't, so evaluate first ;), and you can flag as well to send it up in the flag queue. It helps if multiple people flag the same thing.
Once a question is flagged in this manner, moderators AND 10k+ users are notified, and will usually take action, though we moderators will take action more slowly to allow the community to do its thing.