It isn't going to be another Xehanort doing the same thing. It's still the same old Xehanort. Think of it this way. Young Xehanort goes forward in time because his future self brought him there. As long as he returns Master Xehanort can still exist. It's one person but he is just moving through time. Since Young Xehanort forgets everything when he goes back he just becomes Master Xehanort.