one vs one and only

one is a noun but one and only is not a noun.

Word	Noun	Adjective	Verb	Adverb
one	Yes	Yes	No	No
one and only	No	Yes	No	No

As adjectives, one and one and only are synonyms defined as:

Other synonyms of one include matchless, nonpareil, peerless, unmatchable, unmatched, unrivaled, unrivalled.

one (noun)	one and only (noun)
a single person or thing
the smallest whole number or a numeral representing this number

one (adjective)	one and only (adjective)
used of a single unit or thing; not two or more	eminent beyond or above comparison
being a single entity made by combining separate components
(informal) very; used informally as an intensifier
eminent beyond or above comparison
indefinite in time or position
of the same kind or quality
having the indivisible character of a unit