Looking at the nominal roll, there are the following men named Robert Duncan (splitting them into South Island and North Island for you)
South IslandSurname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address C/- Mrs Seed Southbrook Rangiora
Occupation Farm Labourer
Recruiting District North Canterbury
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address Loburn
Recruiting District North Canterbury
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address 38 Burke St Sydenham
Occupation Farrier
Recruiting District Christchurch
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address Hampden
Occupation Farmer
Recruiting District North Otago
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address C/- D Ross Palmerston
Occupation Shepherd
Recruiting District North Otago
Surname DUNCAN
Given Name Robert
Category Nominal Roll Vol. 2
Regimental Number 24987
Rank Bombardier
Next of Kin Title R
Next of Kin Surname DUNCAN
Next of Kin Relationship Father
Next of Kin Address Menzies Ferry Southland
Roll 37
Page 26
Occupation Bank Clerk
Surname DUNCAN
Given Name Robert
Category Second Reserves
Last NZ Address 24 Greenock St Dunedin
Occupation Mill Hand
Classification C
Surname DUNCAN
Given Name Robert
Category Second Reserves
Last NZ Address Blackball
Occupation Miner
Classification C
Surname DUNCAN
Given Name Robert
Category Second Reserves
Last NZ Address 36 Bath St Christchurch
Occupation Telegraph-lineman
Classification C
Surname DUNCAN
Given Name Robert
Category Second Reserves
Last NZ Address St Kilda
Occupation Painter
Classification F
Surname DUNCAN
Given Name Robert Harold
Category Nominal Rolls Vol 4
Regimental Number 72099
Rank Private
Next of Kin Title P
Next of Kin Surname DUNCAN
Next of Kin Relationship Father
Next of Kin Address 38 Richardson Street Opawa Christchurch
Roll 86
Page 19
Occupation Bank Clerk
Surname DUNCAN
Given Name Robert Shand
Category First Reserves
Last NZ Address Oturehua Maniototo
Occupation Labourer
Recruiting District North Otago
Surname DUNCAN
Given Name Robert William
Category First Reserves
Last NZ Address Nightcaps
Occupation Labourer
Recruiting District Southland
North Island Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address Matarawa
Occupation Farmer
Recruiting District Wairarapa
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address 17 Maarama Crescent
Occupation Cashier
Recruiting District Wellington
Surname DUNCAN
Given Name Robert
Category Nominal Roll Vol. 2
Regimental Number 17282
Rank Gunner
Next of Kin Title R
Next of Kin Surname DUNCAN
Next of Kin Relationship Father
Next of Kin Address 17 Maarama Crescent Wellington
Roll 41
Page 22
Occupation Clerk
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address 9 Knigges Avenue
Occupation Bootmaker
Recruiting District Wellington
Surname DUNCAN
Given Name Robert
Category First Reserves
Last NZ Address Eureka
Occupation Farmer
Recruiting District Waikato
Surname DUNCAN
Given Name Robert Dalzell
Category Nominal Roll Vol. 2
Regimental Number 10796
Rank Private
Next of Kin Title A
Next of Kin Surname DUNCAN
Next of Kin Relationship Father
Next of Kin Address New Zealand Railways Auckland
Roll 31
Page 5
Occupation Ironmonger
Surname DUNCAN
Given Name Robert Wishart
Category Second Reserves
Last NZ Address 156 Grafton Rd Auckland
Occupation Pastrycook
Classification A
West Island (Yeah, yeah, Australia

)
Surname DUNCAN
Given Name Robert Wishart
Category Nominal Rolls Vol 4
Regimental Number 81744
Rank Private
Next of Kin Title Mrs G
Next of Kin Surname DUNCAN
Next of Kin Relationship Wife
Next of Kin Address 38 Blue Street Sydney NSW Australia
Roll 87
Page 13
Occupation Motor-mechanic
As well as that, there are a few with Robert as a middle name. However, all three of these are from the North Island.
Surname DUNCAN
Given Name Charles Robert
Category Second Reserves
Last NZ Address Whangamarino
Occupation Farmer
Classification E
Surname DUNCAN
Given Name Charles Robert Herbert
Category Second Reserves
Last NZ Address Paeroa Thames
Occupation Clergyman
Classification C
Surname DUNCAN
Given Name John Robert
Category Nominal Roll Vol. 3
Regimental Number 59194
Rank Rifleman
Next of Kin Title Mrs E
Next of Kin Surname DUNCAN
Next of Kin Relationship Mother
Next of Kin Address 8 Roebuck Road Gisborne
Roll 72
Page 28
Occupation Drover
Allie