expression
unique; matchless; unparalleled; never again
Used to describe something so exceptional that it is unlikely to occur again. Often modifies nouns like 機会 (opportunity) or 幸運 (good fortune). The literal meaning is 'there is not a second time'.
これはまたとないチャンスだ。
This is a unique opportunity.
彼はまたとない才能の持ち主だ。
He has unparalleled talent.
またとない幸運に恵まれた。
I was blessed with matchless good fortune.