Instructions
Weld together the separate pieces of the prop you wish to duplicate. This is done by selecting the Advanced Duplicator -- which will appear as a gun in your character's hand -- and shooting every separate piece of the item. For instance, if you created a box out of six metal plates, you need to shoot each plate with the Advanced Duplicator.
Save the item to your computer's memory. With the Advanced Duplicator in your character's hand, right- click on the welded item to use. This will duplicate it. Once it's duplicated, bring up your main menu and press the "Save" button. The game will ask you to name it.
Bring up your main menu, and select your saved prop. When you return to the game, a see-through image of the prop will be hovering in front of your Advanced Duplicator gun. When you fire it by left-clicking, you will place the prop, duplicating it.