Is it permissible for a Muslim who has performed Witr prayer with the Imam in Tarawih prayer to pray one rak'ah and then perform Witr again, or should he pray as many rak'ahs as he wishes without Witr?

If you wish to pray supererogatory (nafl) prayers after Tarawih and Witr, then pray as many as you wish, two rak'ahs by two rak'ahs. Do not pray Witr again, because there cannot be two Witrs in one night. The hadith, "The night prayer is two rak'ahs by two rak'ahs. And when one of you fears the coming of dawn, let him pray one rak'ah, which will make the number of rak'ahs he has prayed odd (Witr)," applies specifically to one who has not prayed Witr earlier in the night.