Right now, I have the same sort of issue importing a mesh with created and assigned in BJS materials (pbr, std, multimat). Each time I dispose of a mesh and next reimport it assigning the same material, I get a duplicate (of the material and texture). I don’t know if it only shows as such in the inspector (because of the delta or anything) or if it can eventually impact performance. Now, I’m pretty sure this topic already had its answer somewhere in this forum, but it’s faster and easier for me to just soak up the input that will come for your case 
Following…
Edit: @shaderbytes , sry just found this post of yours:
Do you have any further input/advise on this? Thanks in advance. Cheers,