when i use /stackmodifier i get this in console 

Failed to call hook 'IOnPlayerCommand' on plugin 'RustCore v2.0.5297' (ArgumentException: An item with the same key has already been added. Key: fishingrod.handmade)
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000fb] in :0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in :0 
Failed to call hook 'IOnPlayerCommand' on plugin 'RustCore v2.0.5297' (ArgumentException: An item with the same key has already been added. Key: fishingrod.handmade)
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000fb] in :0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in :0 ​

It's erroring due to a key duplication inside the config file.
It tells you right there at the end of the first line.
Not sure how it managed to end up like that
Just backup the config ( just in case ).

But to fix this, delete the config file and reload the plugin should auto-sort its self out /

(ArgumentException: An item with the same key has already been added. Key: fishingrod.handmade)
​

Thank you that fixed it :)