Tips for unstackable items?

I've noticed that people find ways to stack unstackable items and using them to duplicate the NBT data for those modded items which works because those items don't have anything that handles cases when the items are stacked, they copy data to the whole item stack. How would I go about handling unstackable items in a way that is kinda like this:

recieveNBTData

if(itemStackAmount > 1) {

    copy 1 itemStack to another slot and set stackSize to 1(if there's no room, drop the item)

    remove 1 from itemStackSize of original itemstack

    itemStack = the new itemstack

}

setNBTData for itemstack //Itemstack is either the held item or the new item stack made if the held itemstack is over 1 item in size.