Is it permissible to suffice with a single wash when in doubt about its sufficiency in removing impurity, or is it obligatory to add to it?

Waswas (obsessive whisperings/thoughts) are a great evil, and giving in to them leads to an even greater evil. It is sufficient to immerse the area of a ritual impurity with water for it to become pure. As for a substantive impurity, one must ensure the removal of the impurity itself. A small amount of it that is difficult to remove is excused. It is sufficient to have a strong presumption that the impurity has been removed; certainty is not required, because requiring certainty in this case would be a hardship, and hardship is negated by Islamic law.