I have a model with some split edges that I want to merge: Both vertices and edges are duplicated along the selected edge loop. You can see the duplication in there being two vertex normals (blue) at each point. However, the model contains custom split normals and they are set up so that the vertex-per-face normals (magenta) coincide, giving the surface a smooth appearance.
I want to merge these split edges while preserving the vertex-per-face normals. When I select two vertices at one point of the edge loop and use the merge vertex tool on them, everything works as expected: The two vertices are merged into one and the vertex-per-face normals are kept as they were.
However, when I try the same thing on an adjacent pair of vertices, something strange happens: The vertices are merged just fine. However, the vertex-per-face normals of both the newly merged vertex and the one merged before are affected in a strange way, now being slightly tilted away from each other.
Why does this happen and how can I prevent it? The model contains both smooth (coinciding vertex-per-face normals) and sharp (differing vertex-per-face normals) split edges that I want to merge while preserving the vertex-per-face normals. I have tried both the vertex merge and the vertex remove doubles tools. Both affect the vertex-per-face normals in the way outlined above.