“Duck”, or “ducks” or “duckie”, is quite a distinctive regionalism in England. Your great-grandfather was very likely from Derbyshire, Nottinghamshire, Leicestershire, or somewhere nearby in the East Midlands. “Duck” is used as a greeting even between two unrelated men. Etymologically it is related to the word “duke” and has nothing to do with the bird.
My grandfather came from Ireland but, according to my mother (who knew him as a man in his 80s), he spoke with a Cockney accent! She thinks he must have been in the army with a bunch of Londoners. He served under Kitchener.