一一 (yī yī) means 'one by one' or 'one after another', emphasizing each individual item in a sequence. It is more neutral and flexible than 逐一 (zhú yī, formal/written) and 各个 (gè gè, which also means 'each and every' as a pronoun). Unlike 纷纷 (fēn fēn, many people/things in succession) or 先后 (xiān hòu, focusing on order), 一一 simply describes doing something to each item in turn.