To duplicate any wideo on your account, simply click the "clone" button on the wideo viewer page. This will make a new copy of the wideo on your account.
If you would like to copy a wideo from another user's account, click the "reuse" button. If there is no "reuse" button, the wideo has probably been marked "non-reusable" by its owner. For more information on privacy settings, click here.