This was a long time ago i did this but i will try to explain it ... ... ...
Pokemon Red Duplicate Items
To get 99 of any item, first put the item you want to multiply in the sixth item slot. Now use FLY and go to Viridian City. Talk to the old man almost all the way north of the town. When he asks if you are in a hurry, say no and watch him catch a weedle.
Right after that, fly to Fuchsia city. Make your way south and, when you you reach the water's edge, use SURF and make your way to Seafoam island.
When you get to seafoam island, swim up and down the beach until you run into a Pokemon called "M" (Missingno.) Once you see him, kill him or run away. DO NOT CATCH HIM! After that, you should have multiple copies of your sixth item!
Pokemon Red Glitch: Dulpicating Item Trick (Again)
Put the item you want to copy in the 6th slot in your pack (example: MASTERBALL x1, not POKEFLUTE). Then talk to the old guy in Viridian who shows you how to catch Pokemon and answer "no" and FLY to Cinnibar (or trade w/ a trader on the island). SURF up and down just off of the right (opposit of left, not wrong) side of the island until you find a MISSINGNO or an M-BLOCK. DO NOT CATCH THEM!!! Just RUN instead. Then check your items. The 6th slot will say x(glitch)9. (after the first 2 glitches, just keep using that item. It will go down to x0, then down to x99.)
Priorities:
1.MASTERBALL
2.RARE CANDY
3.NUGGET
http://www.cheatscodesguides.com/gameboy-cheats/pokemon-red/