Here is a test prop that has three walls each with a window.

The first is four quads, completely unwelded. It is 16 vertices.

The second is also four quads, but identical vertices have been welded. The "this" we've been talking about. It is 12 vertices.

The third is two 6-gons and two quads, completely welded. It is also 12 vertices.

All three render identically in Poser, with or without smoothing enabled.

Do they behave the same in other renderers? 

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)