i-adjective
unique; matchless; unparalleled; never again
Describes something so exceptional that it is unlikely to occur again; often used in set phrases like 又無い機会 (a once-in-a-lifetime opportunity). The 'never again' nuance is more contextual than a standalone meaning.
これは又無いチャンスだ。
This is a unique opportunity.
彼は又無い才能の持ち主だ。
He has unparalleled talent.
唯一 means 'only one' or 'sole', focusing on singularity, while 又無い emphasizes the exceptional quality and rarity of something that may not come again.
二度とない literally means 'not a second time' and is used for events that will not happen again, similar to the 'never again' nuance of 又無い, but 又無い carries a stronger sense of value and uniqueness.
Compound of 又 (mata, 'again') + 無い (nai, 'not exist'), literally 'not again'. The exact historical derivation is uncertain, but the meaning has been conventionally associated with the idea of something so rare it will not occur a second time.