"Duck soup" is apparently an idiomatic expression (which I too had never heard) for "something easy to do".
https://en.wiktionary.org/wiki/duck_soup
It doesn't have any note about being regional or restricted to a particular demographic, but seeing as it's apparently a reference to an old movie, it's possible it was more common before and died out.
Without going to the page on Tatoeba for the translations, you can't tell on Jisho which direction the translation originally went. That is to say, someone may have added this English sentence that contains an idiom, and then someone added a Japanese translation that uses a more straightforward expression. It's also possible it was J>E and the person just likes this idiom. Tatoeba sentences are all user-created.
Duck Soup?
No clue what went wrong in the translation, but I don't think duck soup appears in there at all.