This issue can be reproduced by adding both this mod and Metallurgy 4: Reforged in the same instance

This issue is related to:
#1
Davoleo/Metallurgy-4-Reforged#130

I looked into it, but we're already running a check to make sure duplicates materials aren't loaded, the problem is that that check seems to fail when playing with this mod

and the only reason I can think of is that GregsConstruct loads after Metallurgy meaning that when our mod adds Manganese there shouldn't be any other version of it

You can look at crashes and logs in the issue I linked here above