You're probably doing one of two things:
On the purl rows, especially for Continental knitters, the first stitch can look like two stitches, because the yarn is pulling up both sides of the stitch below the working stitch, and you may think these are two stitches that each need knitting! I demonstrate this in the new Continental purling video that I'll be uploading today or late tonight.
Also, you may be forgetting to slip the old stitch off the old needle, and knit it again! I got caught on video doing this a couple of days ago (you'll see my error in the new video on Double Knitting that I'll be putting up tonight! LOL).
We welcome all questions, and that one's a great one!
KnittingHelp Queen Bee
"There is a vitality, a life force, an energy, a quickening that is translated through you into action, and because there is only one of you in all of time, this expression is unique. And if you block it, it will never exist through any other medium and it will be lost. The world will not have it. It is not your business to determine how good it is nor how valuable nor how it compares with other expressions. It is your business to keep it yours clearly and directly, to keep the channel open. You do not even have to believe in yourself or your work. You have to keep yourself open and aware to the urges that motivate you. Keep the channel open."
— Martha Graham