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!