If you use the API to remove an enchantment from an item, then re-add that same enchantment with a different level, then call getEnchantments() on that item the following error is thrown:

https://hatebin.com/rbhqaqvqqn

If you drop the item and pick it back up the error does not re-appear.  Thus it appears that maybe the old enchantment isn't properly being removed by removeEnchantment()

code example:

ItemStack is = plr.getInventory().getItemInMainHand();
is.addEnchantment(Enchantment.DURABILITY, 2);
is.removeEnchantment(Enchantment.DURABILITY);
is.addEnchantment(Enchantment.DURABILITY, 3);
if(is.getEnchantments() != null)
    system.out.println("found enchantment");
 

The above sample would simulate adding an enchantment to an itemstack, then removing it, then adding back the same enchantment with a different level.   This seems to leave two entries with the same key in the map, when getEnchantments() is called the error is thrown, this error will be thrown every time getEnchantments() is called until the item is dropped then picked back up, (it may also fix itself if the user logs out/in again).