Definite accusative case ;)
' because Duo is a proper noun
y because Duo ends in a vowel
u because of vowel harmony (u after o or u, ü after ö or ü, ı after a or ı, i after e or i)
Duo -> Duo'yu, çay -> çayı, kaplumbağa -> kaplumbağayı, kuş -> kuşu, …
I'm wondering the same thing.. and whether this will be in a future lesson or not..