_Hi Hi Paul,
I use Scotland's People a lot, but Duncan does not show up on the 1851 Census. I have sent a query into Scotland People. Ideally finding Duncan's death record could give us his parents and the possible connection.
Apologies as you probably have this already, I have found an Angus McAulay 26 (plasterer born Edinburgh) & Catherine McAulay 22 (Power Loom Weaver born Eastwood Renfrew) at 6 Catherine Street in the Parish of Calton (Glasgow) in the 1851 Census, but no children