Talk to the Old Man in Viridian City and watch him catch a Weedle, then Fly to Cinnabar Island and choose which item you want to duplicate, by putting it in your sixth item slot. Now, this is a bit foggy for me, but surf along the brown long square on the right of Cinnabar Island (you know, the brown strips that seperate land mass from water) and you either have to battle a glitch Pokemon, or capture it in order to duplicate your item.
I can't believe that you haven't heard of how to do it before.
