I'm trying out a fairly common material setup, mixing a diffuse and a glossy shader with a fresnel node. My issue is that my material has some backfaces owing to its construction. They render correctly using either shader on their own, and if I look at their normals plugged into an emission shader they appear to be treated correctly.
However if I use a fresnel node, the results come out strangely, as if the backfaces normals are being treated as flipped (I know of course they really are, but surely this should be ignored when rendering?).
Here's an image showing the issue:
Left: Mesh with some flipped normals. Middle: Normal output of geometry node plugged into emission shader. Right: Fresnel node plugged into emission shader.
I know in most circumstances the solution would be to recalculate the normals, but the mesh I'm working with is a thin sheet that you see both sides of. Is this a bug or can anyone suggest a workaround?