This trick allows you to duplicate any item. (Even limited items such as Langrisser or Runestones). The limitation is that the cloned item will have to stay equipped.

To start, equip said character with desired item to clone. Beat a scenario. Save the game, and go back to the load menu.

Scenario Select back to a scenario where said character appears as a NPC.

You'll still have the item the character equipped in your inventory. This is the cloned item. Equip that item to whoever you want. This action will swap the cloned item with the real item. Now, once you start the scenario, the NPC will be holding the cloned item, and the real item will be equipped to whichever character you gave it to.

Once the NPC joins your party, the NPC will retain the cloned item.

Make sure you don't unequip the cloned item, or it will disappear.