There are so many different ways of darning a sock. Duplicate stitching is definitely one of them and makes a beautiful darn that is almost invisible. So, Godwin is doing it right. You might want to ask this question on Knitting Paradise, a knitting and crochet forum. It's http://knittingparadise.com.

I JUST discovered your books and have been having a delightful time reading them. I'm up to number 3!