The duplicator is a secret accessory in N Terraria 6 that allows the player to place blocks and furniture without consuming them, allowing the player to get infinite blocks. It also makes the player not consume bullets when shooting them, not consume potions when drinking them, it will allow the spawn bosses without consuming the summoning items and so on.
It drops from end-game bosses, examples of possible boss-mobs which can drop this are: Lunatic Cultist, Solar Pillars and the Moon lord.
Tip: You can duplicate armor pieces with the duplicator by right clicking an existing armor piece on a mannequin. This works because placing armor on mannequins counts as placing blocks.