1) Click on "Manage Inventory" under the 'Inventory' heading of the Control Panel.
2) Search for your item by it's Item ID (or Author / Title)
3) Scroll down to the 'Search Results' and click the 'Edit' link under your item.
4) Click on the 'Clone' Button towards the top right of the page.
5) Click the 'Yes, clone new item' button in the pop up window.