Should a prostration of precaution be added when in doubt about the number of prostrations, or should one base it on certainty?

You are afflicted with waswas (whispers/obsessive doubts), and your duty is to completely disregard it and pay no attention to it until Allah cures you of it. So, if Satan whispers to you that you performed only one prostration instead of two, consider them as two prostrations, because scholars have stipulated that a person afflicted with waswas is not obligated to act upon the lesser of two possibilities. You will not sin by disregarding this waswas, but you might sin by indulging in it. Sheikh Al-Uthaymeen said concerning someone who frequently doubts during prayer: "He should not pay attention to this doubt nor prostrate for it, because frequent doubts lead to waswas."