piece of cake vs duck soup

piece of cake and duck soup both are nouns.

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

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

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

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