Reimporting creates new Material Slots instead of overwriting.

CHANGELIST: 2114974
 
BRANCH: UE4 - Main
                        
DESCRIPTION:                             
Using the Reimport LOD1 drop down in the Static Mesh Editor will create new material slots each time instead of overwriting the existing ones.  

AH Post: https://answers.unrealengine.com/questions/60837/material-duplication-after-reimport-lod.html

1.  Open the QAGame Editor
2.  Import a Static Mesh and place the static mesh in the level
3.  Open the imported Static Mesh in the Static Mesh Editor
4.  Under LOD Settings >> LOD Import, Import another Static Mesh into LOD Level 1
5.  Notice the numer of material slots
6.  Under LOD Settings >> LOD Import, Reimport the same Static Mesh from Step 4 into LOD Level 1
7. Look at the Details Panel for the selected Static Mesh.
  
RESULTS:
New Material Slots are created after each reimport
 
EXPECTED:
Existing Material Slots are replaced and no new slots are generated.

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