Witr is part of the night prayer, and it is Sunnah and it is the end of the night prayer, one Rak`ah with which one concludes the night prayers at the end, or in the middle, or at the beginning of the night after `Isha.

Pray Witr with the Imam, then you can pray Tahajjud after that and pray whatever Allah wills for you, two by two Rak`ahs, without repeating Witr. Another option is delaying Witr until the end of the night.

There is nothing wrong with repeating the same Surah in the Tarawih prayer, or in any other prayer. A person may recite a Surah in the first Rak`ah and repeat the same Surah in the second.

The time for Tarawih begins when the ‘Isha prayer is over. There is nothing wrong with praying ‘Isha behind one who is praying Tarawih.