Yep! same here - I would have translated it differently had they not given options:) I don't think I've seen a Latin sentence on Duolingo that uses Duo the owl as a noun, but they do in the French course which I'm also taking, so my mind is kind of boggled:)
Duo declined is... duo.
Only the 3 first number changes according to the gender.
Unus Duo Tres
I give only the accusative here: