Lightning Problem?

While I was creating an artwork in Overwatch’s London and wanted to recreate a real-life railing on Westminster Bridge, I’ve encountered this when I’ve duplicated vertices of the right part and resized it to -1, or mirrored it without using Mirror modifier.

I just want to know what is this and what causes the left part of the mirrored railing part to be “lightened” but the second, which I’ve mirrored not to be lightened, and how to fix it? I don’t think it’s related to UV map, because I’ve tried making New UV map, but with the same result.

Is this some kind of Mesh Lightning or Emission problem after Resizing it to -1?

EDIT: I just figured out it’s name but I don’t know how to fix it yet. The Mirrored object is lighter than the original.

Your normals are facing the wrong.