What is the ruling on someone who doubted the last Tashahhud after Maghrib prayer, so he repeated it, then thought a lot while repeating it, so he repeated it again? Does the prayer become void by the intention to repeat it after finishing it or at a later time? Is it permissible to repeat the prayer again? And what should someone who experiences many of these thoughts during their prayer do?

From what was mentioned in the question regarding the abundance of whisperings (waswas), we advise ignoring them and not paying attention to them, as that is the most beneficial cure for them. If one's doubt becomes frequent, to the extent that it occurs every day, even if only once, then one should ignore it and not act upon what was doubted, and no prostration of forgetfulness (sujud sahv) is required. Re-performing the prayer in the two aforementioned cases is neither legislated nor necessary; for excessive thinking during prayer does not invalidate it, and khushūʿ (devotion/humility) is not among the conditions for the validity of prayer. Doubt about the occurrence of something that invalidates ablution (wudu) during prayer does not invalidate it, because the default is the continuation of purity. Hesitation in interrupting the prayer and then resolving to repeat it later does not invalidate it. If thoughts become frequent during prayer, strive to rid yourself of them, and do not repeat the prayer in this case, because repeating it without a compelling reason is disliked (makruh).