Is it permissible to repeat Surat Al-Ikhlas multiple times in a single rak'ah after Al-Fatihah during the Night Vigil Prayer (Salat Al-Layl)?

There is no harm in repeating a Surah in a single unit (rak'ah) of prayer. It is also befitting for one who has memorized a portion of the Qur'an to recite it in their prayers to review their memorization and to gain the reward of recitation.