Drop the item that you're trying to dupe on the floor in front of you, so you can pick it up

Option 1 - Easy
1) Spam "E" with a macro/script

Option 2 - Requires 2 methods of input: Keyboard/Gamepad
1) Hit "E" on 2 keyboards at the same time. / "E" on the keyboard and the item pick up button on the gamepad at the same time.

Option 3 - Hard?
1) Press "Esc"
2) Press "Esc" again to unpause and press "E" almost instantly after you hit "Esc" check out the timings in the video below at 1:42

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After that you'll have to do different things depending on what item you're trying to duplicate.
Most of the time you just have to drop the item that you duped and re-enter the room, so the game forgets about the original+duped item relations.

For more info + explanation check out the video.

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