bagginsbill is correct- I tested the one-sided square using a Diffuse_Color of black and a Diffuse_Color of white and I got the same results: a mirror without any streaks.
But, it sounds like the issue has been solved. :)

I agree with ockham- two-sided objects must be banned! Poser renders objects as double-sided, so putting textures on both sides of one polygon is just asking for rendering problems.

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