What is the ruling on performing the obligatory prayer twice? Does the second prayer invalidate the first? And what is the ruling on one who performs the obligatory prayer and its time ends?

The Prophet, peace and blessings be upon him, forbade repeating an obligatory prayer twice in one day, unless the second time is with the intention of it being a supererogatory (nafila) prayer, in which case it is permissible. If someone has performed the obligatory prayer and then finds a congregation praying, it is sunnah (definition) for him to pray with them, and his prayer will be a supererogatory one. If someone repeats the prayer as a precaution due to doubt about the validity of the first, and then it becomes clear that the first was valid, then the first is the obligatory prayer and the second is a supererogatory one. However, if he is certain that the first was invalid, then the second prayer suffices for it, even if it was intended as a precaution. Whoever catches a complete unit (rak'ah) before the time of the prayer expires has caught it as a timely performance (ada'an), but whoever does not catch a complete unit, their prayer is considered a make-up (qada').