Duncan takes on the role on an interim basis, having been a director for the FCO's diplomatic service.
The move comes after the FCO ditched the job in November 2011 amid wider cuts to its comms spend of £3m.
Andrew Whyte, who held the role at the time, moved to become a director covering new ways of working at Defra. He has since joined Kingsnorth Communications.
Duncan joined the FCO in 1980 and for the past five years had been the UK's ambassador for multilateral arms control and disarmament.
A spokesman said: 'The FCO has decided to reinstate the post of comms director.
'This will focus on new challenges and opportunities for the FCO and the UK abroad in the wake of Britain's Olympic success. Duncan's role will include continuing the GREAT campaign and enhancing the use of digital comms.'
PRWeek revealed on 9 November that Andrew Pike had been brought in from the British Embassy in Brasilia to help lead comms work around the GREAT campaign.
A full-time appointment would be made in due course, the spokesman added.