I am using a duffel (or duffle) coat again, I would think it is my third during many years. I know it was widely used by the British Navy during WW2, and I remember seeing sailors on u-boats using them when recognizing from the tower. If that was the same kind as you can buy in shops today, they must have been cold. I don’t find my coat very good in cold and windy weather, it is too open in front. Maybe it is differently made than those of the navy people. But for walking around town it is very suitable.
The name Duffel is the name of a Belgian town where the heavy woollen material was made. Read more about it here.