It is not possible to clone or duplicate items in Pokemon Ruby. Duplicating items and cloning Pokemon can only be done in Pokemon Emerald by exploiting the Battle Tower cloning glitch.
The Battle Tower cloning glitch in Pokemon Emerald can be exploited by a player to clone six Pokemon and a large number of items at once. This glitch can only be used for cloning once the Elite Four has been defeated to unlock the Battle Frontier. Once it has been unlocked, head to the Battle Tower and follow the steps below to duplicate an item.
Enter the tower and proceed to Lanette's PC on the far right.
Choose the Pokemon desired to be cloned and have it hold the item wished to be duplicated. Place the Pokemon on an empty box, close the PC and save the game.
Go to Lanette's PC again and withdraw the Pokemon. Go to the woman on the far right desk and enter two Pokemon for battle. The game will pause for a few seconds then she will ask the player to save the game again.
Instead of saving, restart the game. After restarting, the Pokemon that was deposited on Lanette's PC along with the item is still there, as well as the Pokemon with the item in the player's game bag. Repeat the steps to duplicate more Pokemon and game items.