See video first
I’ve been having this problem in which a material is automatically duplicated when apply. I was able to pin down an replicate the issue after several trials.
First of all, it only happens with materials that have transparency. When a material with transparency is created and applied to surfaces or groups works as expected. But when the file is saved, SketchUp closed and reopened again, if you try to apply the material again to a new surface or group it will automatically create a duplicate of said material and apply this duplicate instead.
Despite not shown in the video I have tried to replicate it with all extensions disabled, also applying from one single face to another single surface. Same thing. It will always happen as long as the material has transparency and you had opened the file after the material was saved previously.
I have been using SketchUp for 13 years now. But I just recently started to use SketchUp on mac because of my new job and I find the material management very poor in general compare to the windows version. Is there any plans to improve it?