How do you duplicate items in Pokemon Yellow?
You will need:
~ A Pokemon able to defeat a Lv. 0-255 Bird / Water type Pokemon
~A Pokemon that knows Surf
~ Item(s) on your person that you want to duplicate
MissingNo Glitch [Do NOT catch MissingNo in Pokemon Yellow, the game WILL glitch and you WILL have to start over again.] :
Edit: You CANNOT do the missingno. glitch or the Pokemon above Lv. 100 in Pokemon yellow because Nintendo deleted it so you can't duplicate items that way. ( But they didn't delete the Mew glitch :D )
Go to Cinnabar Island (do not defeat or catch any Pokemon on the way to Cinnabar), and enter the building of the man that makes Pokemon from fossils. In the first room, there will be a few NPCs that want to trade Pokemon. You MUST have the Pokemon needed for the trade; I recommend you save the Pokemon for these trades in order to perform the item trick a few times.
[Make sure you have the desired item to be duplicated in the 6th slot of your list] Trade with one of these people, and once finished, go outside and immediately walk to the right to the shoreline. Use Surf, and travel up and down the shoreline; half on water/half on land. Stay on the shoreline and continue to surf until you get into a battle. It will be with MissingNo, a "Pokemon" that resembles a microchip or barcode. Defeat this Pokemon, (remember that he's a Bird/ Water type, and NOT to catch him.) and open your items list. Notice that the 6th item now has a random symbol next to it instead of a number. This means that the item is [temporarily] infinite. Then, go to the PokeCenter and to your computer. Deposit x99 of that item into your computer, and repeat as many times as possible/desired. After this is done, SAVE YOUR GAME, and repeat (can be done three times all together).