As far as I remember, Flintheart Glomgold was introduced as a stranger to Scrooge in the first comic to feature him, where Scrooge and Glomgold measured their wealth to see who was the richest duck in the world and ended up measuring the lengths of their balls of string to decide. That was "The Second-Richest Duck" by Carl Barks in Uncle scrooge # 15, September 1956.
And there wasn't any clue that Scrooge and Glomgold had any kind of backstory before their meeting in 1955 in his next appearances in "The Money Champ", September 1959, and "So Far and no Safari" in January 1966.
Decades later, Don Rosa in The Life and Times of Scrooge McDuck Part 6 "The Terror of the Transvaal" depicted young Scrooge and Glomgold meeting and fighting each other in the South African Gold Rush of 1886. Though Glomgold learned Scrooge's name, Rosa was careful to be sure that Scrooge never learned Glomgold's name and so didn't recognize that scoundrel when he met his billionaire rival in the 1950s and 60s.