If one has a doubt in prayers for which it is obligatory to perform ṣalāt al‑iḥtiyāṭ, in the event that he completes the prayer, the recommended precaution is that he should perform ṣalāt al‑iḥtiyāṭ; he should not start performing the prayer again from the beginning without performing ṣalāt al‑iḥtiyāṭ. If he starts performing the prayer again from the beginning before he does something that invalidates prayers, then based on obligatory precaution, his second prayer is also invalid.
However, if he starts performing the prayer again after doing something that invalidates prayers, his second prayer is valid.