1) Click on 'Manage Inventory' under the 'Inventory' heading in the Control Panel menu. 

2) Use 'Find by ID' or 'Keyword Search' textbox to find the Item. (Or you can use 'filter' icon next to keyword search textbox for 'Advanced Search')

3) Click the 'Edit' icon at the end of your item to go to it's settings page.

4) Click on '...' (three dots) button next to the 'SAVE' button at the top right corner.

5) Select 'Clone Item' from the dropdown menu.

6) Click the 'YES, CLONE NEW ITEM' button in the pop up window.