Is it permissible for a Muslim to perform supererogatory prayers after the Witr prayer?

There is no harm in praying after Witr for one who performed Witr at the beginning or in the middle of the night. It is recommended that Witr be the last prayer of the night. Witr should not be repeated, based on the Prophet's (peace be upon him) saying: "Make the last of your prayers at night Witr," and "There cannot be two Witr prayers in one night." Witr at the end of the night is better, and prayer after it is permissible, but Witr should not be repeated. The Prophet (peace be upon him) indeed prayed two rak'ahs after Witr.