He co-produced two films of the late 60s with his friend Len Deighton. On the second of these, "Oh! What A Lovely War!", Deighton was so incensed by changes to his screenplay made by director Richard Attenborough that he had his credits as writer and producer removed. Duffy retained his producing credit (Attenborough now being credited as co-producer in Deighton's place), but was also very bitter about the whole experience, and he made no more movies. Only a few months before his death, more than forty years later, he was still speaking with obvious contempt about Attenborough in a TV documentary.