I frequently use duplicate stitch in my knitting. I use it to reinforce any seaming I do, and to reinforce thumbs on gloves and mittens, it's also a good alternative for new knitters who are still scared to attempt other colorwork methods, like Fair Isle or intarsia, to put a design on a piece of knitting after it's completed. Duplicate stitch isn't difficult, all you'll need is your project, a tapestry needle, and a similar weight of contrasting yarn. Duplicate stitch, in my experience, only works on stockinette pieces of knitting. When you're looking at stockinette stitch, you'll see that the individual stitches look like little tiny "v's."
1. Thread a tapestry needle with contrasting yarn. (Don't thread too much onto the needle, it's better to have a few more tails to weave in when finished than pulling feet and feet of yarn through your work, and having a tangled mess on the wrong side of the fabric.) Insert the needle into the base of a stitch, right where the two legs meet at the bottom of the "v." Pull the yarn through, leaving a 6 inch or so long tail.
2. Now insert your tapestry needle into both legs of the stitch above the stitch you originally pulled your constrasting yarn through. Pull the yarn through the two legs, but not too tightly. Just as with knitting, you'll find a tension that works for you with practice of this technique, like Ma' always said "Practice makes perfect." And something about patience, what was that again?
3. Re-insert your needle right back into the base of the stitch that you originally started in, and now you'll see how the contrasting yarn rests right over the two legs of the stitch, effectively masking the original color. I like to think of it like I'm drawing an upside down triangle over the stitch, using the legs of the stitch about it.
In the end, you should have something that looks like the picture above, neat little duplicate stitches square over your original work. Experiment with this technique to create beautiful motifs, or finished objects that will withstand the forces of nature.
As always, if you have any questions, would like a PDF version of this tutorial, or have any requests you can contact me at Ashley.Mishmoshcrafting@gmail.com