I've done a bit of experimenting with these coincident faces/opposite winding orders, and the odd effects of rendering artifacts this causes. Hopefully there's a picture attached to this post showing my results.

I made a simple prop (obj,pp2,texture) with 28 vertices, 8 texture vertices and 12 faces.
It appears as six stacked items, top to bottom:

One face - square, planar.

Two faces - square, planar, same vertices, opposite winding orders.

One face - quad, non-planar.

Two faces - quad, non-planar, same vertices, opposite winding orders.

Two faces - rectangular, planar, at 90 degrees to each other (2 vertices shared).

Four faces - you can work it out...

Each face is texture mapped to make it easy to work out what's what on the renders.

The phrase "SINGLE FACE" on a red background

"DOUBLE FACE (FRONT)" on green

"double face {back}" on yellow

I've numbered each render - here's the key:
1 - Poser6, with default settings
2 - As render 1, but with "Normals Forward" set for the prop.
3 - As render 1, with "backfaceCull" set to 1 in the .pp2 file for the prop.
4 - As render 1, close-up to show the effect on the middle face.
5 - DAZ|Studio 3 render. Similar effect, very obvious on the warped face.

Suffice to say that in every case, two coincident faces with opposite winding orders cause a problem.
Neither "Normals Forward" nor "backfaceCull" seemed to have any noticeable effect, which was a big surprise.

The last two renders were done to try and understand "Normals Forward". The Poser Reference Manual states:
"Checking the Normals_Forward checkbox will flip the surface normal to point towards the camera (or towards the ray in case of raytracing). Use this option to avoid shading artifacts on doublesided polygons."

Renders 6 and 7 show that it does something. These are the props for these two renders, top to bottom:

Single square face

Single square face rotated 180 degrees about the y-axis

Two square faces, same vertices, opposite winding order.

Two square faces, same vertices, opposite winding order, rotated 180 degrees about the y-axis

Texture map:

"FRONT" in red on a white background

"BACK" in blue on a white background

 
6 - P6, default settings, but with a reddish lamp to light up the front, and a bluish lamp for the back.
7 - As render 6, but with "Normals Forward" set.

Any thoughts or comments anybody?

Next step, try English Bob's displacement suggestion.

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).