ArgumentException An element with the same key already exists in the dictionary

Symptom

You see an error on-screen or in log file like:

ArgumentException: An element with the same key already exists in the dictionary.

Cause

This occurs when a mod tries to store a value as a key in a dictionary lookup table, but the dictionary already contains a key with that value.

It can be caused by errors in the mod, or sometimes having multiple versions of the same mod subscribed.

Solutions

In the log file, look for the full error message which will usally start with the ArgumentException error and then be followed by several lines starting with at ....

If you see FineRoadAnarchy.ModInfo:.ctor() or FineRoadTool.ModInfo:.ctor() in the error message, it is almost certain that you have eitehr two versions of Fine Road Anarchy and/or Fine Road Tool mods subscribed. To fix, completely unsubscribe the older versions of the mod (don't just disable them).