How can you duplicate in Pokemon Yellow?

Answer 1

If you are talking about cloning Pokémon, I'm sorry, but there's no way to. That's a glitch/bug introduced in Generation 2. (Gold/Silver/Crystal)

If you're talking about cloning items (such as the Master Ball or Rare Candies), you're in luck, because there's a really easy way to.

1) You need a team that has Pokémon that knows Fly and Surf. You also need access to Cinnabar Island.

2) Move the item that you want to duplicate to the sixth slot of the menu. You press "select" to move items.

3) Fly to Viridian City, and talk to the old man in the northern part of the city. He's the one who says "Ah, I've had my morning coffee..." When he asks you if you are in a hurry, press "No" and watch him catch a Weedle.

4) Immediately after that, fly to Cinnabar Island, and surf along the right coast of the island until you encounter a Missingno. or an 'M. You might run into a few other random Pokémon, like Mewtwo, but just run away and keep on looking for a Missingno. or 'M.

5) After you've found a Missingno. or 'M, you can either battle it, run from it, or catch it. Once the battle is over, check your bag, and there should be 128 or so of your 6th item.

This might sound a bit complicated, but it's really easy once you actually try it.

Hope I helped!