Does repeating the intention before or during prayer invalidate it, and does prayer become void by re-reading a verse from the Qur’an or some of the words of the Tashahhud during it?

We advise you to turn away from waswasa (whispers of Satan, obsessive thoughts) and not to pay attention to them. If Satan whispers to you that you did not make an intention, or did not recite a verse, or did not utter a phrase, then proceed with your prayer and assume that you did so. Repeating a verse or a phrase does not invalidate the prayer, but indulging in waswasa will make your life one of hardship and distress. Waswasa is dispelled by seeking refuge with Allah and by being firm upon the truth, and by repelling these whispers by not responding to them, as Sheikh Al-Islam Ibn Taymiyyah said. Repeating the intention before prayer has no effect on its invalidity. As for repeating it during prayer, it does not harm according to what some Shafi'is have explicitly stated, and this opinion can be acted upon until Allah cures you of waswasa. However, the definitive decision to interrupt the prayer during its performance does invalidate it.