William Robert Duncan (born 14 July 1957) is a former Irish international rugby union player from Northern Ireland.
Born in Belfast, Duncan was a Malone flanker, capped twice by Ireland in the 1984 Five Nations. He made his debut against Wales at Lansdowne Road, then lost his place for the next fixture, before winning a recall at Twickenham when Fergus Slattery had to withdraw with illness.
Duncan played on the Ulster team that defeated the 1984 Wallabies at Ravenhill.