Fly to Viridian City. now put the item you want to multiply in the 6th item slot. Go talk to the old man that shows you how to use pokeballs. Answer NO. After he is done fly to cinnabar island and surf up and down the side of the land by the gym.
This cheat takes patience and luck so it might be tricky. Okay, here it goes. Do the duplication trick.Keep surfing along the coast of Cinabar and if your lucky, youll find a trainer that looks like a scientist named CHIEF in the water! He has a Rhydon level 62, a Magmemite level 62, and a Goldeen level 62. Magmemite has a attack called TM05. ( it upps his defense ) Goldeen knows waterfall.
Level 140 Snorlax:
You can catch this big guy where you catch Missingno.
When duplicating an item, you can avoid having to repeat the process by staying in the water and switching items there. It works as many times as like.
Missingno. Glinch (Level 80):
Use the following steps to see Missingno.
1. You should have your Pokémon that knows Surf and one that knows Fly.
2. Fly to Cinnabar Island.
3. Go inside to the Pokémon Lab and trade with another trainer.
4. Then go outside and surf on the right side of the island and surf so that you are half on land and water.
5. Surf up and down, and you should come up to fighting Missingno at level 80.
Important Info on the Missingno. Glinch Warning! Catching Missingno. could damage your saved game! I don't recommend you trying to catch it, but the only known thing that it does mess up is your Pokémon Hall of Fame.