It's usually in the first stitch of the row. If the yarn goes over the top of the needle to the back, it pulls up the first stitch and makes it look like 2. To prevent that, hold the yarn out to the side a bit so you can clearly see 1 stitch at the beginning of the row.
Look at this sticky thread on the How To forum for other ideas -
http://www.knittinghelp.com/forum/sh...ad.php?t=32886