Please attach a screenshot of each tab. The only way you can end up with 2 is if you install CB Forums then you end up with a tab from CB Forums and a tab from Forum integration. Check CB > Tab Management for the tabs and disable whichever you don't want.
Also, what's the difference between the "Board Catagories" and "Main Forum"? I would like registered users to only have the "Main Forum" as an option (as seems to be the case with the Joomlapolis support forum), how can I accomplish this?
Don't understand the question. See Kunena configuration for configuration of Kunena and creation of board categories.