Codes (Guaranteed to Work)
Missingno. & ' M (Mysterio) - First of all you must have HM 03 (Surf). Then go to the old man in Viridian City that wouldn't let you pass earlierin the game. Talk to him and he will say, "Ahh, Iv'e had my coffee now and I feel great! Sure you can go through.". Then he will ask you, "Are you in a hurry?". Say NO. Then he will say, "I see you are using a PokÚdex. When you catch a pokemon , PokÚdex is automatically updated. What? Don't you know how to catch PokÚmon? I'll show you how to then." Then watch him catch a weedle. After he is done talking surf or fly to either Cinnabar or Seafoam Island. Next surf up and down the shore where it looks like you are halfway on land and half on water. Eventually you will get into a battle with Missingno. or Mysterio!
Item Duplication - *To duplicate any item, you must first have the item with you and it must be the 6th item on your list. Then go into a battle with Missingno or Mysterio and select RUN. Then the the item you wanted will be duplicated.
* The item you use must be an item that says (name of item) x (any #) after it.In other words it must be an item that you can have more than 1 of. Example: PokÚball x 15
HINT:TRY USING IT ON MASTER BALLS, RARE CANDIES,AND MOON STONES.