Mirror and object

Hello everybody, im new to this forum and to the 3d modelling in general, so i will probably ask some obvious questions, hope you guys will have patience to help

so i need to mirror an object and use this model in-game

i tried to use mirror in both object and edit modes, but i keep getting the same issue
when i use this mesh in-game lighting on it is inverted (if light falls on the front side, back side gets illuminated and vice-versa, like if object not mirrored de-facto, only just appear mirrored)

i tried flipping normals, faces, facing normals, everything, all i managed to get is this mesh with lighting fixed, but in that case dynamic shadows used in game are not appearing on it, so it is no good either

i found similar topic on this forum, where user had the same problem and last message reads this:
“fixed! I tried duplicating it again, only I just mirrored the duplicated armature and not the hand.”

Mirroring an object along an axis puts a negative object scale on that axis. ctrl+A -> scale to apply the object scale, which flips the mesh normals, so edit mode, select all, ctrl+N to get the normals fixed.

yes, ive also tried that already, forgot to mention (found somewhere on the other forum)
it doesnt help and messes up model pretty bad also for some reason
applying scale makes all kind of triangular corner shapes appear on the mesh, and fixing normals makes it even worse
dynamic shadows still not appearing after that

im using latest blender 2.7

update: sorry, i guess i was doing something wrong
i did manage to work it this was finally, although it seems i did the same thing as the last time (???)
thanks, i think it works fine now:)