Have two Hunters. The first Hunter should have an open item slot and the second Hunter should have the item to be duplicated. Make sure that you save the second Hunter before going into battle. Go into battle, have the first Hunter attack the second Hunter. Then, have the second Hunter surrender the item to be duplicated. Finish the battle. Without saving the second Hunter, reload him. Now both Hunters should have the same item. It is best to duplicate something such as the Unit X because your can sell it for $15,000 and level up faster.
Press L1 + L2 then hold R1 + R2 at the title screen. Wait for the sound, then let go of the buttons. Press Left, Right, L1, L2, Left, R1, R2, Right.
When creating a character, enter the names SHUICHI or VIKEIF to play as secret characters. Shuichi has a high movement range, decent attack power and HP, but low defense. Vikeif has high attack power, decent movement range and HP, and fairly low defense. They have very good stats for Level 1 characters, but you cannot decide where each stat point goes on level up.