duck soup vs piece of cake

duck soup and piece of cake both are nouns.

Word	Noun	Adjective	Verb	Adverb
duck soup	Yes	No	No	No
piece of cake	Yes	No	No	No

As nouns, duck soup and piece of cake are synonyms defined as:

Other synonyms of duck soup include breeze, child's play, cinch, picnic, pushover, snap, walkover.

duck soup (noun)	piece of cake (noun)
any undertaking that is easy to do	any undertaking that is easy to do