That can be done once you have access to the Mirage Tower, in the desert. Equip your main character with the gear you want to duplicate and talk with the manager, saying you want to challenge the tower in Single Player mode. In the Preparation Screen, choose Menu Screen and unequip all the items from your character. Then leave to the Preparation Screen again. Once there, hit the arrow in the lower right tip of the screen and you will leave the place. Now, check your character and he STILL will be equiped with the gear you removed and your inventory will also have a copy of them.
Mirage Tower
Mirage Tower consist of 50 floors with random monsters. This dungeon allows you to play online with other Ragnarok Online DS players. This dungeon is found at the lower right portal of the North Sograt Desert.
Skill Point Duplication
Go into the mirage tower and save with at least 1 skill point before you enter. Use the skill point then enter the mirage tower and die. When it asks you to save say no and when you appear in the mirage tower you have 1 skill point.
Unlock Geffen
Find the Blacksmith after defeating Giant Elder Willow and access it via West Prontera Field.
Unlock Juno/Yuno
Meet with the Scientist and access it via Airship option on the Prontera Kafra.
Unlock Lighthalzen
Access it via El Mes Plateau.
Unlock Morocc
Complete the Coal Mines, have Viseruk join your party and access it via Sograt Desert (Southwest of East Prontera Field).
Unlock Prontera
Defeat Mastering and access it via East Prontera Field.
Duplicate Equipments...