Ichi-ichi（いちいち）and Hitotsu-hitotsu（ひとつひとつ）
Both “ichi-ichi” and “hitotsu-hitotsu” sound similar , but they are very different.
ichi-ichi (いちいち）
This word is an adverb that means every time, and it has a negative connotation. If you use this word, you sound like you don’t like the action or annoyed with the action. This word mostly means “every time” rather than “one by one”.
It is used like these:
・He annoyingly corrects my misspelling every time he finds.（Karewa watashino tsuzuri matchigaiwo ichi-ichi naosu.)
hitotsu-hitotsu（ひとつひとつ）
This also means one by one, but it has rather a positive connotation. If you add this adverb, it sounds like the action was taken place in a polite manner. This word is not used as “every time”, but “one by one” only.
For example:
・I looked around the paintings one by one. (Watashiwa kaigawo hitotsu-hitotsu mite mawatta.)
