1871 census has George and Anna Duncan at 51 Denby Square, Onchan
1861 census has George and Hannah (sic) Duncan in Darby (sic) = Derby
1851 census has George and Hannah (sic) Duncan in Cheetham, Manchester with children Georgiana (10) and Charles (7)
In all cases George is Civil Engineer
By 1881, no trace, nor of suitable candidates in earlier Scottish censuses.
Origin each time is just Scotland but Ann/Anna/Hannah was born in Halifax, Yorks