adverb, noun which may take the genitive case particle 'no'
one by one; separately; in detail
Used for doing something to each item individually, or for examining things carefully one at a time. Can function adverbially or as a no-adjective.
問題を一つ一つ丁寧に説明した。
I explained each problem carefully, one by one.
一つ一つの言葉に気をつけて話してください。
Please speak, paying attention to each and every word.