DUPLICATE STITCH
I use duplicate stitch more and more often when I know that stranded color knitting will cause problems for a particular pattern. Usually this means that the floats are too long and I can't leave them be because the item I'm knitting is for a child. Duplicate stitch is also helpful in intarsia projects for diagonal lines (example - the diagonal lines in an argyle pattern). Duplicate stitch is not elastic and often feels like a patch on your knitting so keep that in mind if you're planning on using a lot of it on a project that needs to stretch. Some knitters use duplicate stitch on worn areas of sock toes and heels.
When I duplicate stitch, I always go from right to left. I look at my chart and start
at the right side of the first row of the chart and stitch away. When I get to the next row (at the left hand side of the first row), I turn the work around and go from right to left upside down on the next row. Then right-side up on the next row and so on, always stitching from right to left. If you change direction while doing duplicate stitch, you may end up with twisted stitches.
I always do my duplicate stitch from bottom to top. The Stuever intarsia book states there is only one instance in which you'd go from top to bottom - when you have a single vertical line of duplicate stitch that is not near any other duplicate stitched areas.
I generally use a yarn piece about 18" long. I've noticed that if I use a longer piece, it ends up getting caught somehow during the stitching. My tendency is to stitch too loose as doing it too tightly will cause the back color to show through. I take care to try not to split any stitches while doing it as well. Also, when working in the duplicate stitch ends, I make sure not to do too much pulling of the backs of the duplicate stitches which can distort the stitches.
In duplicate stitch the stitches you make (if you're using the same weight yarn as the original knitting) will be larger than the stitches on the original knitted fabric. Fortunately the larger stitches cover up the stitches behind it better.
If I make a mistake in duplicate stitch (I made one in the above elephant pattern from Debbie Bliss' Kids Knits for Head, Hands, and Toes) I correct it by simply duplicate stitching over it. This is generally more pleasant than ripping out duplicate stitch.