also: ひとつづつ
adverb, noun which may take the genitive case particle 'no'
one by one; one each; one at a time
Used when items are handled individually, distributed one per person/thing, or done sequentially.
See also: ずつ
お菓子を一つずつ配った。
I handed out the snacks one by one.
問題を一つずつ解決していきましょう。
Let's solve the problems one at a time.
ずつ is the distributive suffix meaning 'each' or 'per'; 一つずつ is a specific instance meaning 'one each' or 'one by one'.
Compound of 一つ (one) and the distributive suffix ずつ (each).