OBJ + MTL file export material duplicate with "_NONE" suffix

Hi,
I have trouble understanding how to properly export materials in OBJ format. I got everything as i want except that some of the materials are duplicated with suffix “_NONE”.
For example:

newmtl dark_lead
Ns 96.078431
Ka 0.325000 0.325000 0.325000
Kd 0.394010 0.394010 0.394010
Ks 1.000000 1.000000 1.000000
Ke 0.000000 0.000000 0.000000
Ni 1.000000
d 1.000000
illum 2
map_Kd sheet_metal.jpg

newmtl dark_lead_NONE
Ns 96.078431
Ka 0.325000 0.325000 0.325000
Kd 0.394010 0.394010 0.394010
Ks 1.000000 1.000000 1.000000
Ke 0.000000 0.000000 0.000000
Ni 1.000000
d 1.000000
illum 2
map_Kd sheet_metal.jpg

This other material called “dark_lead_NONE” is not needed and I’m not sure how it registers. Blend file is cleared of any other materials and is reloaded. Has anyone came across this issue and found workaround?

Open Data-Blocks in the outliner to see if you can locate that unwated material in your scene.