There are no known glitches or exploits that allow for item duplication in HeartGold.
If you are looking for Rare Candies, here are some legitimate ways to get them in the game:
* Trade: Trading with other players is the most common way to get Rare Candies.
* Events: Occasionally, Nintendo would host events offering Rare Candies as rewards. However, these events are no longer active.
* Breeding: Some Pokémon, like Chansey and Ditto, have a chance of hatching with a Rare Candy as a held item.
Remember, playing fairly and enjoying the game as intended is the best way to experience Pokémon HeartGold!