Here are the rulings of Ayatollah Sayed Sistani in regard the one who has Kathiratu Shakk

Ruling 1170. An excessive doubter is someone who doubts excessively, i.e. a person who doubts more than usual when compared with other people who are like him in terms of being subject to the same factors that cause one to have an unsettled mind. An excessive doubter is not only someone who has already made a habit of doubting excessively; rather, it is sufficient for one to be in a state of developing a habit of doubting [for him to be considered an excessive doubter].

Ruling 1171. If someone who doubts excessively doubts whether or not he has performed an obligatory component of the prayer, he must assume he has performed it. For example, if he doubts whether or not he has performed rukūʿ, he must assume he has performed rukūʿ. If he doubts whether or not he has performed an act that invalidates prayers – for example, he doubts whether he performed ṣubḥ prayers as a two rakʿah prayer or as a three rakʿah one – he must assume he has performed it correctly.

Ruling 1172. If a person doubts excessively about a particular act of the prayer such that his excessive doubting is considered to be only with regard to that particular act, in the event that he has a doubt about another act of the prayer, he must act according to the instructions concerning that doubt. For example, if someone who doubts excessively about whether or not he has performed sajdah also doubts whether he has performed rukūʿ or not, he must act according to the instructions concerning that doubt; i.e. if he has not gone into sajdah he must perform rukūʿ, and if he has gone into sajdah he must dismiss his doubt.

Ruling 1173. If a person always doubts excessively in a particular prayer – for example, in the ẓuhr prayer – such that his excessive doubting is considered to be only with regard to that particular prayer, then, if he doubts in another prayer, such as the ʿaṣr prayer, he must act according to the instructions concerning that doubt.

Ruling 1175. If a person doubts whether or not he has become an excessive doubter, he must act according to the instructions concerning doubts [and not consider himself to be an excessive doubter]. Furthermore, as long as someone who is an excessive doubter is not certain that he has returned to a state that is normal among people, then, if his lack of certainty about this stems from being unsure about a change having taken place in his condition rather than from a doubt in the meaning of being an excessive doubter, he must dismiss his doubt [and consider himself to be an excessive doubter].

Ruling 1176. If an excessive doubter doubts whether or not he has performed a rukn and dismisses his doubt but later realises that he had actually not performed it, in the event that he has not started to perform the next rukn, he must perform that rukn [about which he doubted] and what follows it. However, if he has started to perform the next rukn, then based on obligatory precaution his prayer is invalid. For example, if he doubts whether he has performed rukūʿ or not and dismisses his doubt, in the event that before performing the second sajdah he remembers he has not performed rukūʿ, he must go back and perform rukūʿ; but if he remembers this in the second sajdah, then based on obligatory precaution his prayer is invalid.

Ruling 1177. If a person who doubts excessively doubts whether or not he has performed an act that is not a rukn and dismisses it and later realises that he had not performed it, in the event that the time for performing that act has not passed, he must perform it and what follows it. However, if the time for performing it has passed, his prayer is valid. For example, if he doubts whether or not he has recited Sūrat al‑Ḥamd and dismisses his doubt, in the event that he remembers in qunūt that he has not recited Sūrat al‑Ḥamd, he must recite Sūrat al‑Ḥamd and the other surah; but if he remembers this in rukūʿ, his prayer is valid.