Why is my mirrored object in blender lighter/darker than the original object? Even with a simple cube? EDIT: I Figured it out! 
I just hollowed out part of the mesh, on the side that would touch the mirrored object. 

Hopefully this can help others having the same problem.

Dunno how that worked, but it did.



Original post:

Whenever I try to mirror an object, the mirrored object is always lightor or darker than the original.

For example...


Is there a way to fix this? Originally I was using this mirror method to create a lightbar (below):



But as you can see, one side of the triangular bit is CLEARLY different from the other in terms of shading..

However this only seems to be the case on smooth shading, flat shading looks just fine.

However, when I export it, to be used in a game, it reverts to smooth shading and looks awful.
Figured it out! I just hollowed out part of the mesh, on the side that would touch the mirrored object. 

And basically just went, delete / faces on that part. 

Hopefully this can help others having the same problem.
Here is an image showing what I mean. 

http://imgur.com/a/7PIHf