Thanks, Monica. You have the right family. There was one John Duncan in Duke Street, first at 20 Duke Street (1891 census), then at 12 Duke Street (1901 census). The P.O. and Trade Directories seem to refer to 12 Duke Street as 12 Arnsheen Terrace.
John was railway worker for most of his adult life: stoker (1875 marriage register; 1881 census) then engine driver (1891 census; 1899 daughter Jane's marriage register). In 1901 at age 55 he seems to have retired and is listed as Gardener's Labourer (1901 census) then Gardener (1915 and 1920 Valuation Rolls). John is not the first ancestor I've found who retired from labouring to become a gardener.