Chicolini: You don't have to teach me, I know how!
(to prove his point, he kicks him)
Let's face it — trying to single out funny moments from Duck Soup is impossible. The movie is one long Crowning Moment Of Funny from five minutes in when Firefly first enters to the fadeout at the end.
Groucho's favorite line in the film was, "Remember, you're fighting for this woman's honor, which is probably more than she ever did!"
Both times Firefly gets the ambassador to call him an upstart.
The motorcycle and sidecar running gag. The first two times Groucho gets in 'his Excellency's car!', his drive Harpo would go off, leaving the sidecar behind. The third time, Groucho gets wise. ("Oh no. You can only fool a Firefly twice. This time you get in the sidecar.") While the motorcycle remains still, Harpo in the sidecar rides off.
Rufus T. Firefly: This is the only way to travel.
The whole bit where Groucho makes Chico Secretary of War, from start ("You want to be a public nuisance?""Sure! How much does the job pay?") to end with Harpo showing off his tattoo's to Groucho ("Well, I know one thing. I bet you don't have a picture of my grandfather." Harpo starts to remove clothes.)