ok once you have duplicated your geometry or mirrored it before you combine , open the UV texture editor and select the UV's on your mirrored geometry making sure you have the whole shell (it will look in the UV editor as though you only have your original UV's but infact what you have is two sets of UV's directly ontop of each other so they look like one set),so whith the mirooed shell selected change to the scale tool and scale the UV'S in the direction you mirrored ie if you mirrored to -1 in the x scale the uv's in the x until you get a mirrored version of the uv's and of course don't forget to move them from ontop of the other set. you can now if you want combine your geometry and merge the verts.