When someone votes to close a question as a duplicate, they provide a link to the potential duplicate and the system auto-generated a "possible duplicate of ..." comment.
Users who can't yet vote to close can flag the question as a duplicate instead. They also provide the link to the possible duplicate, but there's no auto-generated comment in that case. I think there should be - it would allow others (especially those with enough rep to vote to close) to avoid having to hunt down the link.
It might also be a good idea to include this question as an option in the close-as-duplicate dialog.