The duplicate stitch in knitting, or Swiss Darning, is a decorative stitch that overlays the original stitching of an already knitted piece in a contrasting color, to give the piece texture and dimension. Properly worked duplicate stitches completely cover the stitches they overlay, and are typically worked over stockinette stitches.
To create a duplicate stitch, thread a tapestry needle with the contrasting color of the same gauge or with larger yarn as the original piece. Using a smaller gauge risks not covering the original stitches. Stitch from the top of the "A" of the knit stitch to the bottom of one side before coming back out the top. Next, stitch to the bottom of the other side of the stitch, and come out at the top of the next "A." Continue to work this way until you complete all desired duplicate stitches,.