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Continuity mistake: When Rey is shown for the first time while undergoing training in the forest she falls to the ground face-down. When she gets up her clothes are soiled with dirt as expected, but in all shots after that, in the same forest sequence, her clothes are perfectly clean again.
Answer: Palpatine has been manipulating Rey and Kylo Ren from the very start, bringing the pair together and instigating fights between them. He is trying to make Rey stronger so that when he takes over her body, she will be powerful. Palpatine can partially see the future and knows that Kylo will not be successful in actually killing Rey, but that their constant fights will cause the two to grow more powerful. Because he can see part of the future, he knows that eventually Rey will triumph over Kylo and "kill" him, and then come to confront him directly. He doesn't count on Ben Solo coming back to face him which winds up being his undoing.
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