yup i would just knit two together. as long as you are looking at only one or two extra stitches you won't be able to tell (and as long as you don't do a whole bunch of K2tog right next to each other of course!
as far as how it happened...if you are just doing knit stitches across make sure that when you turn your work you are keeping the yarn under and behind the needles. often when you turn the work the yarn will go up over the needle. when you do that it makes the last stitch on the previous row look like there are TWO stitches there. essentially you are doing a yarn over which creates an extra stitch. just make sure that the yarn is down in front if you are doing purl stitches and down in back if you are doing knit stitches.