Is the re-washing for major ritual impurity (janabah) waived if washing the neck was forgotten, based on Ibn Taymiyyah's fatwa?

If the questioner is afflicted with waswasah (whisperings/obsessive thoughts) and has the slightest doubt about forgetting to wash his neck, then he does not need to repeat his prayer, and he should ignore these whisperings. However, if he is certain that he forgot to wash his neck, then his ghusl (ritual bath) is incomplete, and he must repeat the prayers he performed with this incomplete ghusl. Ibn Taymiyyah differentiates between the one who forgets and the one who is ignorant: the one who forgets repeats his prayer, whereas the ignorant one may not need to repeat it.