I'm assuming your using Maya right? I haven't found a 100% fix yet. I was doing some reading online and apparently Maya has lots of issue's normal mapping when there is mirrored uv's. If you use the hardware render it will fix the problem but then the rendering is kind of bad. The other fix is to just cut your model in half, re-duplicate and mirror but don't weld it back together, which is what I'm currently doing.. Let me know if you any questions.