At the moment links to possible duplicates are inserted in the question text once a question is closed. It would be better to insert the links right away as soon as someone votes for it as a duplicate.
People interested in answers to the question would probably be interested in related questions, even if the current question is not closed as a duplicate yet. Even if the question never reaches the closing threshold, the links to the related questions would be useful. At the moment only users that can vote to close can see these links before the question is really closed.
One can add comments with links to the duplicate questions, but this is not very readable and ends up as duplicate information once the question is closed.