Is it permissible for someone who wishes to be cautious in his religion to repeat prayers and fasting, the number of which is uncertain, for an unspecified number of days, and to offer one or two severe expiations, and is there any guidance regarding that?

If what the questioner is experiencing is merely a whisper (waswasah), then he should disregard it, for doubt after completing an act of worship has no effect and should not be paid attention to. However, if it is not a whisper, and the doubt concerns the original performance or make-up of acts of worship, then it is obligatory to make up whatever prayers, fasts, and expiations one is doubtful about, until the soul is assured that the obligation has been discharged. The Messenger of Allah, may Allah's peace and blessings be upon him, said: "Allah's debt is more worthy of being fulfilled." It is advised not to repeat acts of worship that one is certain were performed, and to hasten in making up any acts of worship, missed duties (fawa'it), or expiations that have not been performed.