Answered by: Alimah Umm Abdullah
I prayed Isha already then went to bed. I woke up later in the night prayed Isha again thinking I didn’t pray. It was when I finished praying that I remembered that I had already prayed before going to bed.
In the name of Allah, the Most Gracious, the Most Merciful
If a person offered his Fardh Salaah and then prayed it again, it will be considered a Nafl prayer. 
Only Allah Ta’ala knows best
Written by Alimah Umm Abdullah
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham.
وَالْكُلُّ يُسَمَّى نَافِلَةً لِأَنَّهُ زِيَادَةٌ عَلَى الْفَرْضِ لِتَكْمِيلِه
رد المحتار على الدر المختار، ج٢/ص٣، المكتبة الشاملة