Ref. No. 983/41-132
In the name of Allah the most gracious the most merciful
The answer to your question is as follows:
You should act upon whatever is certain to you and do Qada Akhira after every rakat that you suppose the last one. If you are confused about the validity of namaz after completion, then it is better to repeat the same.
Repeating the same surah in two rakats intentionally is not liked but the namaz would be valid.
And Allah knows best
Darul Uloom Waqf Deoband