Assalam alaikum,
Does repeating Fatiha or tashahud deliberately sometimes more than once invalidate Salah especially if the person suffers from doubts in nearly every Salah.
Jazakallahu khair

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

This repetition does not invalidate the prayer, but the repetition of Al-Faatihah is disliked as stipulated by the scholars of the Hanbali School of jurisprudence.

On the other hand, the treatment of much doubt that reaches the point of whispers is to not respond to them and give in by repeating what this doubt orders you to do. Rather, the treatment is to ignore them and not pay any attention to them.

Therefore, you should not pay any attention to this doubt and recite Al-Faatihah and the Tashahhud once only until Allah relieves you of this disease.

For more benefit on doubting proper recitation of Al-Faatihah, please refer to Fataawa 177703 and 370603.

Allah Knows best.