After you mirror your shape you need to mirror the UV map as well.
Highlight the faces you mirrored, then open the TCE. Select "flip horizontal" (or vertical if you mirrored it that way instead) and then snap the faces to the other side of the map. Resize the map to fill the entire space space without going over and make sure there are no holes.
As always, the usual rules apply:
http://www.inivis.com/forum/showthread.php?t=5025