I have a problem with my model. Using mirror modifier or duplicating and scaling -1 on the X axis sometimes results in an half darker than the original one. I have recalculated normals and removed doubles but the problem doesn't get solved. I hope you will be able to help me]1
if you'r referring to the different left/right shades your model shows above, imho @Duane Dibbley in a comment to your question has the cause and the solution. I'll try to show some details about how to "solve" this issue.
Your model in 3d view is lit by 3 opengl (invisible) lights, which - by default - are not "aligned" with your model, so they cast light and shadows on your model asymmetrically. This is why you get that impression in 3d view.
Your scene has a "real" lamp object, which will light the rendered view, but also is not "aligned" with your model: if you align it on X, and move a bit in front of your model, the rendered view will be perfectly lit in a uniform and symmetric way (even enabling the smooth shading):
To have the same lighting in the (opengl) 3d view, you have to make the (invisible) opengl lights "aligned" with your model, although it could be a bit difficult, but you can try, and you can also switch 2 of them off: open the user preferences panel, go to "system" tab, and try switching off two of them, and then try to make the remaining one "centered" (this is difficult because you can only operate on a small "disc" with the mouse cursor...). Here an example to give you the idea:
This sure looks like the meshes are simply not fused at some places. If the middle vertices are not in the
Merge Limit tolerance of the mirror modifier, they will not be fused and will remain separate, consequently causing different normals and thus shading on either side. So, you could increase the
Merge Limit value, but I would rather recommend moving the affected vertices into the middle. This will be easier if you enable
Clipping in the mirror modifier. Then you can just drag them towards the middle and they will snap and remain there.
You should keep
Clipping enabled to prevent the middle from accidentally leaving the
Merge Limit zone during transformations.