Is the hadith: "There are not two witrs in one night" authentic?

The Hadith you asked about was narrated by Imam Ahmad in Al-Musnad, and Shu'ayb Al-Arna'ut said about it: "Its chain of narration is hasan (good) because of Qais ibn Talq." It was also narrated by Al-Nasa'i in his Sunan, and by Al-Tirmidhi and others, and Sheikh Al-Albani graded it as sahih (authentic).