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You may be pulling the working yarn strand up when you begin the row. That makes the first stitch in the row look like two stitches and when you work both sts you'll be increasing sts. Try pulling the working strand of yarn down or to the side so that you can see that there's just one stitch there. You might also try a marker near the end (say, 2-4 sts in from the ends). It'll make it easier to count how many sts there should be at the ends. The marker can be as simple as a loop of contrasting yarn placed around the needle between sts and slipped from one needle to the other each time you come to it.
This thread may also help: