What should one do who prostrated a single prostration of forgetfulness instead of two, after discovering that?

If the questioner's sister limited herself to one prostration for the prostration of forgetfulness, then she did not perform it [correctly], because the prostration of forgetfulness consists of two prostrations. The preponderant opinion is that her prayer is valid and is not invalidated by omitting the prostration of forgetfulness, considering that it is a Sunnah, nor is it invalidated by adding an extra prostration due to ignorance or forgetfulness.