Recently discovered something odd regarding the processing of materials it seems. Not sure when this started happening, so I won't be able to pinpoint if it might have been a recent update to the website or not.
Anyhow, I noticed that, when you upload a model with multiple parts that, for instance, uses the same material, when editing the model under the 3D settings, sketchfab duplicates the materials equal to the amount of different parts that use the same material.
In my original .blend file I only have 3 materials, one being the gold plating, the other is the gem, and the last is the pillow on the inside of the casing. However in sketchfab I have multiple gold materials being added.
Here is an example image of what happens with models using the same material in the same file: