If you think the original question is a duplicate of one of the related question or if the related question is a duplicate of another related question, etc., flag them for closure.
What you are doing is for the greater good for the community and the site and I am sure the mods would really appreciate it if you do so too.
The functionality of the related question side bar is to help the questioner to quickly go through related question of the same topic. It is possible that the related question answers the original question, in which case the questioner wouldn't have to duplicate the question at all. But, on occasions, the questioner might not actively check the related questions sidebar and that's when the community steps in and helps close the question if it should be rightfully closed as a duplicate.