Hand animations are duplicated when using spells

Please provide a picture and more info. I can't tell what you mean by that.

Here

If that's the vanilla firstperson arm animation, then you need to add that item to the config to auto switch to vanilla hands/toggle off the mod. If it's actually a duplicated pair of arms of the 3rd person model flying around I'd ask over at the Iron’s Spells ’n Spellbooks mod github page/discord.

What should I add and where?

If these arms are from the firstperson animation (the one you see without this mod), then Iron’s Spells ’n Spellbooks mod is incompatible, so you need to open the config file and add the item ids to the correct lists.