Assalamu aleykum. I have a question regarding the opening dua for prayer (dua thana). I have wrongfully read this dua in first and third rakah of every fard prayer, I recently discovered my mistake as the dua should only be read at first rakah. Are all the prayers I performed like this in my life invalid now? Hope you can answer .Thank you

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.

The prayer is not invalidated by reciting the opening supplication for the prayer in the third Rak’ah, and it does not also necessitate prostration for forgetfulness.

Indeed, the jurists stated that if a praying person recites a legislated Thikr (mention of Allah) in prayer at a position other than its proper place, then it is not obligatory for him to prostrate for forgetfulness; rather, it is only desirable for him to do so.

For more benefit, please refer to Fatwa 349990.

Allah knows best.