Does repeating the word "wa rahmah" twice, due to error or thinking it was mispronounced in the tasleem, invalidate the prayer, and is it obligatory to repeat it for that reason?

Repeating the phrase of peace (As-salamu alaykum) in prayer due to whispers (waswasah) is not legislated, but it does not invalidate the prayer nor necessitate its repetition. This is because exiting the prayer occurs with the saying "As-salamu alaykum" according to the majority of scholars. The person afflicted with whispers must disregard these whispers and not succumb to them.