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Brendon O'Regan's idea for airport duty-free shops came from one on the S.S. America.
sailing on a return voyage from the U.S to Ireland aboard the ocean liner S.S.America. “I discovered that I could buy duty–free goods aboard the ship. I thought: Wouldn’t this be an excellent idea if we could idea if we could implement such a duty-free service at Shannon Airport.” Upon his return O’Regan secured the enabling legislation establishing a no-man’s land tax zone at the airport and the world’s first duty-free shop opened in 1947.
I don't know if there was anything before this but I thought this might be of interest. I don't even know for sure what the date for Brendan O’Regan's trip was but it was obviously before 1947, maybe 1946 after it was returned to civilian service after WW2 (see Wikipedia S.S. America).