Not sure what your problem is... you cannot create groups with the same name in the FOP2 Manager, that is normal/intended.
Creating a group does nothing in particular, unless you assign that group to a user in the Users tab. So, go to the users tab, edit your user, and uncheck any groups it might have, leave it with no groups selected at all. Then save, reload fop2 and try to log as that user, do you see buttons then ?