This is a proposal on changing this procedure... We'll see where it gets. ;-)
Instead of adding a comment mentioning it's a duplicate, answer the question by referring the user to the duplicate answer with a link and only then flag the question as a duplicate.
It takes (moderator) time to approve a duplicate flag and people trying to answer questions. (I regularly see multiple comments to say it's a duplicate)
- The OP has an "answer" immediately
- Other users hunting for unanswered question can see the question has an answer (gaining them time)
- Less unanswered questions on the site
- The answer does not contain any additional useful information
- Duplicate information (kinda, as it's supposed to link to the original answer)
Any other pros or cons you see?