ringer vs dead ringer

ringer and dead ringer both are nouns.

Word	Noun	Adjective	Verb	Adverb
ringer	Yes	No	No	No
dead ringer	Yes	No	No	No

As nouns, ringer and dead ringer are synonyms defined as:

Other synonyms of ringer include clone.

ringer (noun)	dead ringer (noun)
(horseshoes) the successful throw of a horseshoe or quoit so as to encircle a stake or peg	a person who is almost identical to another
a contestant entered in a competition under false pretenses
a person who is almost identical to another
a person who rings church bells (as for summoning the congregation)