Materials leaking into scene? What's wrong?

I have a problem while trying to change/manipulate materials in scene.
I’m getting error about materials leaking into scene.
My idea is to manipulate material during run-time in such a way so user can change main(diffuse)texture in light-mapped object and thus change its look. But when I do this in script after testing in editor I end up with renamed material names containing multiple word instance and all affected objects are black…
What can be wrong?(I’m still learning)

Can anyone provide a link or simple example how to do that right way in light-mapped object?
I have trouble and It’s nearly last thing to solve in my project.

More tech. details:
scene is light-mapped
objects tagged by material type (wood, floor, plastic, etc.)
multiple objects share exact same light-map
various instanced/duplicated objects (chairs, doors, etc.)
when changing material in “instance” other instances should remain intact

Than You for any idea or advice. I already searched forum for 3 days, no luck to find example of this. :evil:

Image to better illustrate what I’m after :slight_smile:
I should mention I find that others also have this problem but nothing about for what to watch for…

353812--12315--$lightmap_example_203.jpg

Any idea?