The SNP administration at Holyrood wants Scotland to be a global leader in the field and match the current UK target to spend 0.7% of income on international development.
Mr Duncan, Conservative MP for Rutland and Melton, said Britain already shapes the global agenda.
"I think the UK as it is leads the world in development and I really think, for the sake of the poor, it would be good to keep it that way," he said during a visit to Edinburgh.
"I cannot see how, by splitting our efforts, it would be the same.
"Arithmetic suggests it could only be less but I would like to keep the structure together in a way that fully includes Scotland.
"This is the point: Scotland is critical to our current fantastic effort and it is an essential component of what we do for many reasons.
"There are many Scottish people in the Department for International Development (DfID) and in the field of development.
"We have an equal headquarters in East Kilbride, there are NGOs and development enthusiasts in Scotland."
A link with his department and Edinburgh University is doing "amazing things" for poor people in Africa.
"I don't want to put that partnership at risk, I'd like to see it grow," he said.
Despite his outlook on the constitution, he said he maintains polite relationships with Scottish Government ministers.
The comments were made during a visit to Holyrood High School in the Scottish capital.
He was there to meet pupils and promote the UK Government-funded "connecting classrooms" programme which works with Lombeta High School in Tanzania.
The project allows students to take part in exchange visits and helps teachers to work closely together.
He wants to encourage more pupils to take part in DfID's "shape the future" competition which calls for ideas on how the Government can make life better for girls and women in developing countries.
Mr Duncan said: "I've been deeply impressed by the enthusiasm, energy and commitment of young people in Scotland who are helping change people's lives in some of the world's poorest places.
"From building links with schoolchildren in a Tanzanian village to raising awareness of HIV and Aids in Malawi, the young people I met in Edinburgh can be proud of the impact they are having in developing countries.
"Young Scots have a hugely important role to play in helping the UK create a better future for people around the world and I hope that as many as possible take up this challenge."
SNP plans for overseas aid were discussed at Holyrood earlier this week.
MSPs on the European and External Relations Committee were told to focus their goals and not spread work too thinly in multiple countries, whatever the outcome of the referendum on September 18.
Scotland already works closely with Malawi, among other countries.
Humza Yousaf, the Scottish Government's International Development Minister, said Mr Duncan is using the poorest people as a political football.
"When I met Mr Duncan yesterday, I asked him to match the Scottish Government commitment to protect the aid budget in law. However, he said he saw no reason to do so despite the Tories' promise to do so in both their election manifesto and the coalition agreement.
"For Mr Duncan to attack Scotland's ability to be a leader on global poverty just after admitting that he is happy with the Tories breaking their promise to the world's poor shows exactly why we cannot trust the Tories on aid.
"Westminster's record on international development is one of 42 years of missing the United Nations aid target and £87.5 billion of missing aid.
"The UK comes behind many countries of Scotland's size in the Centre for Development's Commitment to Development Index. Denmark, Sweden, Norway and Luxembourg top the index with the UK in 8th place after Ireland. That gives the lie to the claim that big is best in this area - and shows what an independent Scotland can aspire to."