It is not prescribed to repeat the recitation of al-Faatihah in one rak‘ah. If this sister failed to focus properly when reciting al-Faatihah, then she realised, she should focus properly in what is left of her prayer. If she does that, she will soon get used to paying attention and continuing to focus properly, in sha Allah.
With regard to repeating al-Faatihah for this purpose, there is the fear that it will lead to waswasah, then she will not offer any prayers until she has repeated al-Faatihah several times. Waswasah is a disease and an evil that one should beware of.
It says in al-Insaaf (2/99): It is makrooh to repeat al-Faatihah. This is our opinion as it is the view of the majority of our companions, and it was stated definitively by many of them. And it was said that it invalidates (the prayer). End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: The words “repeat al-Faatihah” mean repeating it twice or more.
The reason for that is that it was not narrated from the Prophet (blessings and peace of Allah be upon him) that he did that. If a person repeats al-Faatihah as an act of worship, there is no doubt that it is makrooh, because if doing so were good, the Prophet (blessings and peace of Allah be upon him) would have done it. But if a person repeats al-Fatihah not as an act of worship, but because he did not do it in the recommended manner, it seems that it is permissible, such as repeating it because he forgot and recited it silently when it is prescribed to recite it out loud, as happens to some imams who forget and recite al-Faatihah silently. In this case we say: There is nothing wrong with repeating it from the beginning in order to make up for what was omitted of reciting out loud. The same applies if one recites it without focus and wants to repeat it whilst focusing on the second recitation; in this case repeating it has a legitimate purpose, which is to focus with presence of mind. But if there is the fear that this will open the door to waswaas, one should not do it, because for some people, if they open this door they will face a great deal of waswaas and when they recite it, if they are careless in one verse they will repeat it. Then if they lose focus when repeating it, they will lose focus a second time, and a third and a fourth and so on, and then things will become very hard on him.
End quote from al-Sharh al-Mumti‘, 3/331.
And Allah knows best.