The Duplicator is a secret accessory found in the Inverted Castle (specifically, in the Reverse Clock Tower, behind a breakable wall near the top). It costs a staggering 500,000 gold from the Librarian. However, its effect is game-breaking: it allows Alucard to use any consumable item or sub-weapon an infinite number of times without consuming them.
With the Duplicator, you can spam powerful sub-weapons like the Cross or Holy Water, use Soul Steal endlessly, or heal yourself with Elixirs without worry. This makes the game significantly easier, especially for boss fights and challenging areas. Farming gold to purchase the Duplicator is a worthwhile endeavor for any player looking to maximize Alucard's power.