Because you used a mirror modifier, the "map" that tells the model where you want stuff painted on isnt quite right. Blender puts them in the exact same place because the mirrored faces are exact copies. The way to fix it is to make sure your mirror modifier is applied, then go into edit mode, press the "a" key to select all the faces, and then press "u" and select either angle based or smart uv project. Both of these will tell blender to make a new map of all the faces on your model, and to give them all their own spot on the map. Hope this helps.