You actually have to copy the master object s.
def DupeMaster(dupenum):
font = Glyphs.font
master = font.masters[0]
for d in range(dupenum):
print "dupe", d
masterCopy = master.copy()
font.masters.append(masterCopy)
font.copyInfoFrom_sourceFontMasterID_targetFontMasterID_(font, master.id, masterCopy.id)
DupeMaster(3)
The last line is a nice helper to copy all the layers and the kerning.