There is nothing wrong with the person who prays Witr in the first part of the night or the middle of the night offering any naafil prayers he wants after Witr, even though it is mustahabb for the last of his prayer at night to be Witr. In this case he should not repeat Witr, rather the Witr that he prayed at the beginning of the night is sufficient.
It was narrated that ‘Abd-Allaah ibn ‘Umar said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Make the last of your prayer at night Witr.” Narrated by al-Bukhaari, 998; Muslim, 749.
And it was narrated that that Talq ibn ‘Ali said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say, “There should not be two Witrs in one night.” Narrated by al-Tirmidhi, 470; al-Nasaa’i, 1679; Abu Dawood, 1439. Classed as saheeh by Shaykh al-Albaani in Saheeh al-Jaami’, 7567.
Ibn Hazm said in al-Muhalla, 2/92, 93:
Witr at the end of the night is better, but whoever prays Witr at the beginning of the night, that is fine. It is permissible to pray after Witr but one should not repeat Witr.
Al-Nawawi said in al-Majmoo, 3/512:
If a person prays Witr then he wants to offer a naafil prayer etc at night, that is permissible and is not makrooh, but he should not repeat Witr. The evidence for that is the hadeeth of ‘Aa’ishah (may Allaah be pleased with her) who was asked about the Witr of the Messenger of Allaah (peace and blessings of Allaah be upon him). She said: “We used to prepare his siwaak (tooth stick) and water for wudoo’ for him, then Allaah would cause him to wake up at any time He willed at night. He would use the tooth stick, do wudoo’ and pray nine rak’ahs, in which he would not sit except in the eighth, when he would remember Allaah, glorify Him and make du’aa’. Then he would get up without saying salaam, and pray the ninth rak’ah. Then he would sit down and again remember Allaah, glorify Him and make du’aa’. Then he would say a salaam that we could hear, then he would pray two rak’ahs after that, whilst sitting down.” Narrated by Muslim; this is part of a lengthy hadeeth, which is to be understood as meaning that the Prophet (peace and blessings of Allaah be upon him) prayed two rak’ahs after Witr to show that it is permissible to pray after Witr.
And Allaah knows best.