Here are all the middle names used for Duncan found on the 1911 England and Wales census and cross-referenced with the CRB.
Duncan Adam
Duncan Alastair
Duncan Albert
Duncan Albert Herbert
Duncan Aldred
Duncan Alec
Duncan Alexander
Duncan Alfred
Duncan Allen
Duncan Ambrose
Duncan Angus
Duncan Archibald
Duncan Arthur
Duncan Athol
Duncan Aubrey
Duncan Augustus
Duncan Basil
Duncan Bernard
Duncan Bertram
Duncan Brian
Duncan Bruce
Duncan Caesar
Duncan Cecil
Duncan Cedric
Duncan Charles
Duncan Charles Malcolm
Duncan Clarence
Duncan Claude
Duncan Clement
Duncan Clifford
Duncan Colin
Duncan Conrad
Duncan Cuthbert
Duncan Cyril
Duncan Daniel
Duncan David
Duncan Donald
Duncan Douglas
Duncan Douglas Stuart
Duncan Edgar
Duncan Edward
Duncan Edward Curtis
Duncan Edward Frederick
Duncan Elijah
Duncan Eliot
Duncan Ernest
Duncan Evan
Duncan Ewart John
Duncan Fergus
Duncan Frank
Duncan Frederick
Duncan Frederick Basil
Duncan Frederick William
Duncan Geoffrey
Duncan George
Duncan George Leopold
Duncan George Mark
Duncan Godfrey
Duncan Godwin
Duncan Gordon
Duncan Gilbert
Duncan Graham
Duncan Gregory
Duncan Harold
Duncan Harry
Duncan Hector
Duncan Henry
Duncan Henry Frederick
Duncan Herbert
Duncan Horatio
Duncan Hugh Frederick
Duncan Ian Noel
Duncan Ivan
Duncan Jason Joseph
Duncan James
Duncan James Sidney
Duncan John
Duncan Joseph
Duncan Josiah
Duncan Keith
Duncan Kenneth
Duncan Kerr
Duncan Laurence
Duncan Laurie
Duncan Leslie
Duncan Lester
Duncan Lewis
Duncan Lionel
Duncan Llewellyn
Duncan Lloyd
Duncan Logan Andrew
Duncan Malcolm
Duncan Mungo
Duncan Nathan
Duncan Neil
Duncan Oswald
Duncan Patrick
Duncan Paul
Duncan Percival
Duncan Percy
Duncan Randolph
Duncan Reginald
Duncan Richard
Duncan Robert
Duncan Roderick
Duncan Romeo
Duncan Ronald
Duncan Ross
Duncan Samuel
Duncan Septimus
Duncan Silvester
Duncan Stanley
Duncan Stephen
Duncan Stewart
Duncan Theophilus
Duncan Thomas
Duncan Valentine
Duncan Victor
Duncan Vincent
Duncan Wallace
Duncan Walter
Duncan Wilfred
Duncan William
Duncan William Horace
Unsurprisingly, quite a few of the Duncans listed were born in Scotland.
