Strong's G1527 — εἷς καθ᾽ εἷς (adverb)
heis kath heis (heis` kath heis`) adv.
1. individually, one at a time
2. (specifically) one by one
3. (literally) one and one
[from G1520 repeated with G2596 inserted]
KJV: one by one 
Root(s): G1520, G2596 
Hebrew root: κατά