Perhaps this was a second marriage for Hugh?
Hi Catherine
I did a bit a digging earlier re: the Hugh / Margaret MILLIGAN family (just haven't had time to get back here to post the findings.)

I'd also firstly entertained the possibility that this might be a second marriage for Hugh, however after viewing the Probate record of Hugh MILLIGAN junior (d. 1877) I found it contains an affidavit from Margaret Hood MILLIGAN (wife of Hugh MILLIGAN the Elder of Oxford, Canterbury), confirming that she was the mother of Hugh junior.
[Hugh MILLIGAN junior, a Contractor living at Cheviot Hills, died at Christchurch Hospital on 28 February 1877, intestate (i.e. without leaving a will.)
His father Hugh, His mother stated that (
his her son) was a bachelor and did not leave "lawful issue". Hugh (the elder) sought to administer the estate. ]
I'll post a link so you can view the Probate file.
~ Lu
Edit: On re-reading the Probate file, I see that it was actually mother Margaret Hood MILLIGAN who made the statement above ... (have amended accordingly. )