When upgrading a project that was prior to the material slot implementation to a newer engine version will cause redundant material slots to be created. This requires the mesh to have LODs and multiple materials to reproduce.
RESULT
There are additional material slots added that are identical to the preexisting materials.
EXPECTED
A material slot is generated for each material and no additional one are created.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-42731 in the post.