Instructions
Place only two things in your character's item bag ̵1; a Monomate and the item you wish to dupe.
Go to the shopping area. Speak to the woman who talks about Tekkers. Lead her to the armor shop by letting her walk a few steps toward you and then speaking to her, repeating the process until she is as close to the armor shop as possible.
Turn toward the armor shop and press ̶0;A̶1; as soon as the woman is close to the armor shop. Immediately choose ̶0;Buy.̶1; The woman's dialogue text should appear at the same time as the armor shop menu.
Select any item from the armor shop. Hold the analog stick toward the left and press ̶0;A̶1; repeatedly. This moves the camera to the nearby green item shop, displaying its item screen.
Press ̶0;A̶1; once to make the screen disappear. Press ̶0;B̶1; and then ̶0;A̶1; to make it reappear. If this doesn't work, restart the process ̵1; starting with talking to the woman ̵1; until it does. If it does work, you'll be able to walk around the game with the item screen open.
Go to the bank and talk to the bank teller. When presented with the choice of ̶0;Deposit̶1; or ̶0;Take,̶1; choose ̶0;Take." A ̶0;Yes̶1; or ̶0;No̶1; option appears. Leave it for now. Press ̶0;B,̶1; ̶0;B̶1; and then ̶0;A̶1; to access another meseta currency and item screen.
Walk around once again. Press ̶0;B̶1; if the "Deposit" or "Take" screen has disappeared, but don't press ̶0;B̶1; if it hasn't. Press the ̶0;Start̶1; button to bring up the main pause menu. Select ̶0;Item Pack̶1; and then ̶0;Deposit̶1; ̵1; both screens will open.
Select an item you wish to dupe and press ̶0;A.̶1; You'll notice various menu screens are overlayed upon each other. Press the joystick up and down until both the ̶0;Drop̶1; option and the ̶0;No̶1; option from the previous ̶0;Yes̶1; or ̶0;No̶1; option screen are selected. Press ̶0;A̶1; and the item will drop.
Press ̶0;Start̶1; to close the item screen. The deposit window remains open. Select the item in question and choose ̶0;Yes.̶1;
Exit all the windows and pick up the dropped item. Go to the bank to withdraw the duplicated items.