Born in Rio de Janeiro, Duarte served the Brazilian Foreign Service for 48 years.

He was the Ambassador of Brazil in a number of countries, including Austria, Croatia, Slovakia and Slovenia concurrently (1999–2002), China (1996–1999), Canada (1993–1996) and Nicaragua (1986–1991).

In addition, he has been posted as the Brazilian representative to several international organizations, focusing on disarmament issues.

In 2005, he was the President of the 2005 Seventh Review Conference of the Parties to the Treaty on the Non-Proliferation of Nuclear Weapons.

Duarte obtained his Bachelor of Law from Fluminense Federal University (Niterói, Rio de Janeiro) in 1958.