A Mod caused an error [ModException]

Details:
System.ArgumentException: An element with the same key already exists in the dictionary.
at System.Collections.Generic.Dictionary`2[System.String,UnityEngine.Texture2D].Add (System.String key, UnityEngine.Texture2D value) [0x00000] in <filename unknown>:0
at RoadsUnited.RoadsUnited.CreateVanillaDictionary () [0x00000] in <filename unknown>:0
at RoadsUnited.ModLoader.OnLevelLoaded (LoadMode mode) [0x00000] in <filename unknown>:0
at LoadingWrapper.OnLevelLoaded (UpdateMode mode) [0x00000] in <filename unknown>:0
ModException: A Mod caused an error