1
general::
Phrase(s): one by one [and] one at a time
the first one, then the next one, then the next one, etc.; each in turn. • I have to deal with problems one by one. I can’t handle them all at once. • Okay, just take things one at a time. • The children came into the room one by one.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, one at a time. Individually in succession, as in The ducklings jumped into the pond one by one, or One at a time they went into the office. Formerly Also put as one and one and one after one, this idiom dates from about A.D. 1000.
American Heritage Idioms