@Ceciliabr
That's great - it's nice to get one right every now and then.
I've actually used a lot of Kolbe windows in my current project -- but for the most part they look different from yours because I only recently (as in two days ago) realized many of their models in 3DWarehouse are actually composites incorporating a core window design along with a number of casing options, each with its own layer. Unthinkingly, I'd exported my furniture objects with all layers visible -- and then wondered why the models were so needlessly complex. I'm currently in the process of re-exporting all my windows and doors -- correctly, this time.
Most (all?) Kolbe models are double-glazed, so you might want to try hiding or deleting one of the panes before export; if that doesn't fix things, you could eliminate one pane and replace the other with a single face. This should correct the refraction problem while still supporting reflections.
