I have a scene with 5 materials that I exported with the Blender Exporter 6.2.
It turns out that when I opened the babylon scene in the Sandbox, there are 6 duplicates of each materials.
All duplicates except one is actually linked to the mesh (i.e. if I change the color of the material, the color of the related mesh change in the Sandbox viewport).
Here is a screenshot of the Blender scene on the left, and the BabylonJS scene on the right:
And here is the log of the export:
I think that it might be a bug?