Ganbare Goemon Gaiden 2: Tenka no Zaihou

First, go to a village where a Bank building is located. This trick also only works if you have less than 5 Ryou (it costs 5 Ryou to use the Bank to store items). Go there and attempt to deposit the item you wish to copy. You will be stopped since you can not afford the 5 Ryou to do so. Keep pressing A repeatedly and the same message will appear. Each time you go through this message, you will get another of that item. When you leave the place without depositing anything, check your inventory to find your duplicate items!