Normal Map, Tangent Binormal on Mirrored Objects

[shivanangel]

The mirroring takes place when I use the symmetry modifier on the polygonal mesh. In this case, I created a high resolution mesh for 1/8th the roof of a cylindrical building.
I then use the symmetry modifier 3 times (rotating the modifier pivot each time), the first creates 1/4 of the roof, and then 1/2 and finally the entire roof. I had already UV Mapped the roof so that each symmetry would have the same UV coordinates, increasing the space available in the texture map.

There is no tiling actually occurring, all UVs are within the 0 to 1 area.

Attached is an image of the UVs, with a selection made to show how they are mirrored across the 3D object.