Hitler was never a common name. That particular spelling seems to be unique to people descended from Adolf Hitler's father, Alois Hitler.

Alois Hitler was born to an unmarried mother and originally had the surname Schicklgruber. At the age of 39, he legally took the name of his stepfather (who was likely his biological father as well) Johann Georg Hiedler. For unknown reasons, he used the spelling Hitler instead of the more common Hiedler. [1]

[1] Toland, John (1976). Adolf Hitler. Garden City, New York: Doubleday & Company.

Not sure if this answers your question, but Adolf's father, Alois Hitler Sr., was born Alois Schicklgruber. He later tried to change his name to match his step-father (or possibly his father, sources are murky about it) Johann Hiedler, but for unknown reasons, it was misspelled 'Hitler'.