Keep normals when mirror.

For my work (e.g. Anime characters), I often edit the normals, but I encounter issue when I use the mirror modifier, after editing them. If I use the "Merge" option, the center's normals get broken.

I can fix it by duplicating the mesh, collapse the Mirror modifier, fix the normals and use it as a projection on the symmetry center of the mesh that I still have the Mirror modifier with "merge" activated, but it is bothersome, and I want those points to be welded.

So I thought that it will be good to have an option "Keep normals" like the triangulate modifier.