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Joomla does not allow you to install the component twice as it uses same table space and resources. Since you are looking for a reall forum (Answers is not a forum, a Q&A component), you can do this way:
Community Answers component for all people (open). All registered users can ask questions and every one can answer questions as well.
Second component as Kunena (closed). You can set the moderation enabled and all posts can be moderated before they are published.
Please note that the above implementation is mostly similar to what we are using in this website.