Whoever has performed the witr prayer and then wishes to do some more, he may do so but he is not to repeat the witr.
Abu Dawud, an-Nasa’i, and at-Tirmizhi have recorded from ‘Ali that he heard the Messenger of Allah say: “There are no two witr prayers in one night.” At-Tirmizhi grades it hasan.
‘Aishah relates that the Prophet would make the taslim in such a manner that we could hear him and then, he would pray two rak’at while sitting. This is related by Muslim.
Umm Salamah also narrates that he prayed two rak’at while sitting, after the witr prayer. This was related by Ahmad, Abu Dawud, at-Tirmizhi, and others.