Ever Since I Can Remember, My Arm/hand Muscles Have "copied"

Ever since I can remember, my arm/hand muscles have "copied" themselves. Meaning I can write with my right hand and my left will noticeably copy my right. Vise versa. In time my hands have gotten easier to control. I am 21 now. However, when straining one arm; I will get a sharp pain in the other. As if they are linked somehow. If you have any idea what this could be please let me know. Thank you.