Once already we have considered the problem created when Evan Treborn returns to the same events a second time. Having done it to save the dog, he did it again to save the baby at the mailbox. In both of those cases, though, his first visit was as an observer, discovering the events that had happened during the blackouts of his youth. This time it’s different. He had already traveled to that basement to warn Mr. Miller to leave Kayleigh alone, undoing the kiddie porn film and making Kayleigh’s life much better. Now he travels to that same moment, displacing himself and creating a different new version of the past.
All of the problems already addressed apply afresh, but with a new wrinkle: why is Evan not fighting with himself for control of himself? We understand why the blacked out Evan is going along for the ride; he does not know what is happening as it is. We can almost grasp why the first and second trip Evans, being mere observers, do not interfere with the interventions of the fifth and seventh trip Evans. However, the Evan who came back here on his third trip ought to be present, trying to accomplish his agenda, while the Evan on his ninth trip has a very different agenda.
It does not help to say that the actions of the ninth trip Evan will undo those of the third trip Evan. Certainly that is true, both because third trip Evan now will never have made the porn film and because he does not have the journals and has never discovered his ability to travel to his own past. However, that logic will mean that ninth trip Evan cannot make the trip, either, and thus the acts of both of them must be preserved by Niven’s Law if either is to become history. Nor can we suppose, as with fifth trip Evan, that ninth trip Evan got here first; he seems to have targeted the basement more precisely, and perhaps arrived second. We are back to the question of who displaces whom.
The film, though, assumes that whoever makes the last time travel event controls the events in the past. It does not explain, nor even consider, what happens to the traveler who made the previous trip. This becomes one of its mysteries, preventing us from developing a coherent theory of time from it. It also means that whatever this Evan Treborn did has altered every event in which Kayleigh was involved, which is almost every time travel event he ever made, and enough of them that his new self would not have near the same experience. Again, though, he remembers the undone histories, not the newly created ones, so he knows what he needs to know.