I had this problem and it actually only applies to you viewing the forum. Try logging in using another username and you won't get this problem.
Go to Admin> Create Member Permissions (and select your own username seing double) > Delete (bin image) the doubled forums.
And you're done; it doesn't stop you posting or viewing posts there, but for some reason I believe that the admin sees things a little skew-wiff and this definitely fixes the problem. Obviously, I am not sure what it is doing in the underlying database but it does work!