Handle duplicate mods

[2767mr]

If there are two mods with the same name there should at least be a warning. Maybe we should block all but one version from loading? (Only load the newest version?)

[naxane]

We should load neither version and throw an error. Any decision made to load one mod or the other will be arbitrary, as the modloader can make no gaurantees about what the mod does, let alone whether it's compatible with an older or newer version of itself.