Instructions
Take your Ditto to the Pokemon day care center on Island 4 of the Sevii Islands. Make your Ditto hold the item you wish to clone.
Give the daycare center owner Ditto and one of your other Pokemon, and wait for them to mate. This should take about 5 to 10 minutes.
Pick up an egg from the daycare owner after your Ditto has mated with the other Pokemon, and walk round until the egg hatches. The Pokemon inside the egg will be holding the item you gave to Ditto.
Return to the daycare center and pick up your Ditto. This Pokemon will still be holding the item he had when you dropped him off, thus giving you two of the same item.