common duckweed vs duckweed

common duckweed and duckweed both are nouns.

Word	Noun	Adjective	Verb	Adverb
common duckweed	Yes	No	No	No
duckweed	Yes	No	No	No

As nouns, duckweed is a hypernym of common duckweed; that is, duckweed is a word with a broader meaning than common duckweed:

common duckweed: of temperate regions except eastern Asia and Australia
duckweed: any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds

common duckweed (noun)	duckweed (noun)
of temperate regions except eastern Asia and Australia	any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds