From youth, he displayed great musical talent, composing his own original works and collecting and arranging traditional Breton songs.
Duhamel explained his own federalist vision: The current status of Europe is outdated, and the internationalization of economic life requires a political federation where existing states allow room for genuine national communities.
However, copies were seized by the French government on its publication because its perceived anti-French viewpoint was seen as seditious on the outbreak of World War II.
Duhamel died of cancer in 1940 without having completed the second volume of his History of the Breton People, which was to have covered the period after the union of Brittany with France in 1532.
Brittany was central to Duhamel's inspiration as the names of his works such as Impressions de Bretagne and Esquisses bretonnes emphasise.