I'm with theoart. That sounds normal to me, I've never experienced the one-sider any other way. 0 side is invisible in preview, but in render you see the 1 side backwards, as if light were going through it as well as being reflected by it. Every now and then it would be nice to have a one-sider be truly invisible on the far side, but more often the way it currently works is a useful texture-saver.