Duplicate stitch, or Swiss Darning, is taking a second colour of yarn, and "embroidering" on the right side of the knitted fabric by following the path of the yarn that was knitted to create the fabric. So, you would come up from the back at the point of the V in stocking stitch, follow up the side of the V then go down to the back, come up again at the top of the other side of the V, then down at the point of the V. If doing more than one stitch, you would come up at the point of the V in the stitch beside the one you just did, and repeat the actions.
If that description is difficult to follow, my apologies -sometimes is hard to describe in words without pictures or showing it- but you could google for picture tutorials or videos, too :sm01: