I only know that Sennheiser promoted Duofol (diaphragm with two diffrent thicknesses for stiffer dome and softer surrounding suspension) with the introduction of the HD590, in 1999. Quote from Sennheiser's press release: "[size=10pt]Duofol transducer welds two Mylar films to make one rigid diaphragm. This is light and self damping, [/size]reducing resonances in the diaphragm itself and reducing flexing when driven hard."
I think Sennheiser successively implemented this technology in other models as well, which may indicate that there are early and late driver versions of the HD580 and HD600. And maybe the HD250.
Duofol is also mentioned in the marketing of the PX series and the HD555 and HD595.