Commonly used by gumshoes in books by Chandler and Spillane. If something is as good as done or will be done very easily is said to be "duck soup."
This phrase was coined after the 1933 movie of the same title, directed by Leo McCarey, written by Bert Kalmar and Harry Ruby and starring the Marx brothers.
A gathering of Duck Soup is aptly named, because Groucho Marx described Duck Soup... '"Take two turkeys, one goose, four cabbages, but no duck, and mix them together.'
In other words, all the left over or inferior crap thrown in together, but nothing exotic or tasty. thus we are in human terms left with overweight, balding, sweaty men and women, none of whom are attractive, all trying to score each other.
Jo: Its Sunday night Jeffy, they are Duck Soup!
Guy 2: "Man you're so sweaty though"
*Guy 1 performs duck soup*
Guy 1: "What the fuck dude, duck soup is so nasty!"