Van Duijn was born into a theosophical family in the Hague. He attended a MontessoriGrammar School and, subsequently, the Montessori Lyceum, where he attended the Gymnasium He specialised in letters and graduated in 1963. In the Hague he had been active in the peace movement, organizing sit-down demonstrations against the nuclear bomb. He had also been editor for De Vrije Socialist, an anarchist magazine. After graduation he moved to Amsterdam to study political science and history, later turning to law. In 1965 he was one of the founders of the anarchistcounter-cultureProvo movement. In 1969 he was elected into the Amsterdam municipal council for the movement. In 1969 he founded the Green counter-culture Kaboutermovement and was involved in the Oranjevrijstaat. On April 17, 1970 he was briefly abducted by right-wing radical Joop Baank. Although Van Duijn informed the police authorities afterwards and pressed charges against Baank the latter was not prosecuted. In 1973 he became a member of the progressive political party Political Party of Radicals. In 1974 he became Amsterdam alderman for the party. He refused an official car, but instead took an official bike. On February 15, 1975 a bomb was placed in the Venserpolder metro station, which was under construction, by a group of right-wing radicals including Baank. The authorities assumed left-wing squatters had planted the bomb and Van Duijn was the only member of the local government who refused to sign a statement blaming them. His period as alderman ended prematurely in January 1976. During his period as alderman he instigated several initiatives; the use of sustainable energy, a municipal cable network and the municipal television channel. In 1977 he became an organic farmer and started a cheese farm in Veele and had two sons. In 1981 he returned to Amsterdam and in 1983 he sold the farm. In 1984 he was candidate for the European Parliament for the Green Progressive Accord, a combined list of Political Party of Radicals, Communist Party of the Netherlands and the Pacifist Socialist Party. He did not gain a seat, but joined the parliamentary party as a policy advisor. He soon became disenchanted with the European Parliament and the PPR. He founded the local party Green Amsterdam and was elected in the 1986 municipal elections. In 1987 The Greens won one seat in the North Holland Provincial council with support of Green Amsterdam. On March 10, 1989 Green Amsterdam joined The Greens. In 1989 he was their national top candidate, but failed to gain a seat. From 1990 until 1998 he was local councillor in Amsterdam for The Greens. In 1999 he became their provincial councillor in North Holland. In 2001 he joined GroenLinks. He had unsuccessfully pleaded for a merger of De Groenen and GroenLinks. In 2006 he became ward councillor for GroenLinks in Amsterdam Oud-Zuid.
In 2009, it was shown that Van Duijn had been shadowed by the General Intelligence and Security Service for decades. He demanded an apology and access to all documents, but only received access to the documents until 1982. He then filed a lawsuit and an appeal to see the rest.
