Complex sentence: Doise is not the first writer to explore the hypothesis of Esterhazy as a double agent: earlier writings by Michel de Lombarès and Henri Giscard d'Estaing, though differing in the details of their theories, also presented this line of argument.
Split into simpler sentences: Doise is not the first writer to explore the hypothesis of Esterhazy as a double agent. Earlier writings by Michel de Lombarès and Henri Giscard d'Estaing, though differing in the details of their theories, also presented this line of argument.