Duplicate stitch is a way to work a design or pattern into your knitting after the piece is complete. It is actually embroidery worked on the surface of the knitted fabric. It is used to add color details, such as a stripe or small motif, but you can also use it to correct small mistakes.
Bring yarn to the outside at the base of the stitch below the stitch to be duplicated. Pull yarn through to front. Place needle from right to left behind both sides of the stitch above the one being duplicated. Complete the stitch by returning the needle to where you began. When working a series of horizontal duplicate stitches, complete one stitch and begin the adjacent stitch in one step.