So I was thinking:
If you had a friend that was a duck, say a duck that was around person-size, and you were out playing catch with a bunch of your friends (including said duck), and you caught the ball and turned and threw it at your duck-friend, but oh no he wasn't paying attention, and it was headed right for him, what the fuck would you yell at him?
Say his name was Jake.
If you yelled "Jake!" He'd turn and get it right in the face. If you yelled "Duck!" He'd still turn and get it right in the face, this time a little peeved that you had taken to calling him "duck" instead of "Jake." So that's even worse. "Jake, Duck!" wouldn't work out either. You could try something like "Jake, crouch!" But he'd most likely spend the time he should be spending...umm...ducking wondering something like "Crouch? Who says crouch?" and he'd still get hit with the ball.
All in all, that duck would be fucked.
And what were you doing playing catch with Jake, anyways? Dude doesn't even have hands!