"In every case, I would avoid the use of 'double-sided' polygons.  There are just too many ways that reliance upon this will fail or turn out bad results"

I totally agree. I have deleted the "2-sided" square from my props library (in both P5 & P6).
Why on earth it was originally included in Poser is beyond me. And why it continues to be included, given the number of threads here (and elsewhere) that have asked "why am I getting these strange artefacts" just shows that someone in the Poser Development team isn't taking notice...

Normals (the thing which tells the renderer "which way is outward") are, in Poser, calculated from the vertices that make up the polygons ('cos Poser ignores any "normals" info in the .obj, as do many other apps). Normals are calculated using the order in which the polygon vertices are declared (the winding order). So if it finds 2 polys with identical vertices, one or the other will take precedence. And which one takes precedence is arbitrary (it's probably the last "identical" poly that it finds, but I don't really know). The result is, that for any 2-sided surface in Poser, 1/2 the surfaces are ignored, and the resulting normals point every whichaway.
So some are rendered properly and the rest are rendered from the wrong side.

Quite why that affects the P4 renderer in P5 (giving the above-mentioned dark lines) I have no idea, but I'm not really worried, since I now recognise the symptom and know how to deal with it.

For me, the only time it becomes a major PITA is when a 3rd party prop uses them. At least, in P5 that was the case (it meant a trip to my 3D modeller to convert all 2-sided parts of the mesh into 1-sided). At least in P6 that can usually be solved..:-))

Sorry if I'm boring you..

I'll shut up now...

Cheers,
Diolma