When one renames an item in the anvil and shift clicks to put it in their inventory then after closing the anvil UI the item(with the original) comes back into the inventory.
This works for every item in minecraft. For example if you rename a diamond sword to say BLADE and shift click to put it in your inventory, then after closing the UI you will have 2 swords in your inventory, 1. the renamed one, 2. the original one.