here's the problem, when you do knit stitches the yarn is hanging down in the back. when you turn it, it is now in front. if you move it to the back by pulling it up over the needle that first stitch will look like there are two stitches that need to be knit. you have to make sure that after you turn it that you bring the yarn back by going UNDER the needle so that extra stitch doesn't appear there. same thing with bringing it to the front when purling. has to go under.
if you stop and just play with that first stitch you will see what we are talking about. Before you do anything pull your yarn up over and look to see what happens to that first stitch on the needle, then pull it down under. you will see that extra "leg" when you pull it over. THAT is the extra stitch you are likely knitting.