WARNING: ATTEMPT THIS EXPLOIT AT YOUR OWN RISK. YOUR ACCOUNT COULD BE BANNED FOR DOING THIS.

This isn't really duping, it basically creates a phantom of a weapon that you can equip. Once you unequip them, it's gone. This works on any weapon on GL or JP.

What is unique? Once you enhance a weapon it becomes unique. There is only 1 in the world. Even if you or someone else enhance a weapon in the exact same way, their ID would be different.

Requirement:

• At least 1 base weapon of that same weapon.
• At least 2 enhanced weapons of that same weapon.

How to reproduce:

• Say you are trying to create a phantom Bowie Knife with High Tide and lots of HP. Let's call this weapon "David"
• Equip David to a unit
• Save the party that unit is in
• Equip David to a different unit, not in that saved party
• Load the previously saved party
• Now you have David equipped to 2 different units, you can swap them around from the equip menu. A single unit can dual wield David, or you can have 2 units on the same party equipping David.
• Enter a battle and confirm that both David are equipped and functional.

Edit: Title is wrong, you can make more than 1 phantom of a unique weapon by repeating the save/load process.

Edit2: /u/PencilFrog have found a bigger bug from party save/load, I'll just paste his post:

This bug is much more expansive than this. In fact, it extends to all equipment.

Obviously this is a rather exploitable bug. Don't go doing anything your not ready for the potential consequences of.

Video ... I don't know why my screen recorder added black bars, but okay

Steps —

Equip a unit/units with whatever gear you want to dupe
Save the team
Equip the gear you want to dupe on a different unit
Make sure the units that were saved are not in your current active party. They can be in other parties, but not the >party you are about load over.
Select Load
Select the party you saved
View the unit's equipment
Back out