Because of the problem you bring up, ulf, is why some suppose that there are infinite "slices" of time or the ability for infinite "realities" and these "realities" hold every possibility. Therefore, if someone where to go back in time, though he or she had never been in that time before, then a new "slice" or "reality" would be created in which that person was/is in that time. The "reality" in which that person never went to that time and he reality where that person actually did go to that time would co-exist on separate planes of existance.