Ibn Hazm (rh) 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 (rh) said in al-Majmoo, 3/512:
If a person prays Witr, then he wants to offer a naafil prayer and so on at night, that is permissible and is not makrooh, but he should not repeat the Witr.