`RecalculateNormals`

is not robust when double-sided meshes that share vertices between front and back faces are used. It cannot properly determine which triangles should be used to generate the vertex normals in that case.

To properly calculate normals when multiple triangles share the same vertices you must selectively generate the normals based on the triangles you care about.

Inigo Quilez has a nice article on generating mesh normals that I recommend reading. However, you can calculate normals using the example code listed below.

```
normals = new Vector3[vertices.Length];
// This for loop should be adjusted to only process the triangles you actually want to contribute towards the final normals.
// For example, triangleLength should be half the length of the triangles array if you have an extra set of reversed triangle indices for the back faces.
for (int t = 0; t < triangleLength; t += 3)
{
int i0 = triangles[t];
int i1 = triangles[t + 1];
int i2 = triangles[t + 2];
Vector3 a = vertices[i0];
Vector3 b = vertices[i1];
Vector3 c = vertices[i2];
Vector3 n = Vector3.Cross(b - a, c - a);
normals[i0] += n;
normals[i1] += n;
normals[i2] += n;
}
for (var i = 0; i < normals.Length; i#)
normals[i].Normalize();
```

If you are still having issues with your triangles, perhaps the vertex normals are correct, but you have duplicated triangles. Do some visual debugging and check that the winding order of your triangles are correct.