First off you must deposit all of your money and i suggest you bring the item that you plan on duping if you are new to this. Next talk to the lady located by the shops.(the one who tells you about the tekkers) Let her walk toward the armor shop and then talk to her. Once she is finished talking to her right as she turns around talk to her again so that she moves further toward the shop. Once she is pretty close to the shop talk to her and the shop owner at the exact same timeOnce you get that down get the buy armor screen to come up (you should be able to scroll through all the armor). Hold left and keep pressing a until you either appear at the weapons shop or the items shop.

Once there select buy (If you havent selected it already) and press b and then a. You should be able to walk around with the window open. Also make sure that you can scroll through the weapons. Now run to the bank and select take then select items. Once you have done that press a, b, b, a. There should be a window that says Deposit and Take. If not press b. Now press start and select item pack and deposit then select items and items. Two windows should pop up. The one on top is the deposit window and the one on bottom is the items menu. Select the item you want to dupe in the bottom menu. Once you have selected it it there should be no in one window and drop in another. Select both at the same time and you will drop the item. Now press start to get rid of the lower window and select the item you just dropped in the upper one. It should then say yes or no. Select yes and you will deposit the item. Now exit out of everything and one item should be in the bank and the other on the ground.

I suggest you master just plain old duping first because of the fact this is a little more tricky. First you must go through all of the steps above until you get to the depost and take window. Now choose item pack and deposit at the same time. Now you must select item and item at the same time. Select Item, when it says yes/no remove/drop/sort press B twice. Select Deposit and Item at the same time.
select any item and the option Item, Press B.
Select deposit and your meseta on the left at the same time.
Now select meseta and drop at the same time and then
press down to select all your meseta on both options. Press A.
now pick up all your meseta from the ground and check the bank and you should have your meseta duped.