how can I clone an existing form

If the form you're trying to clone is an existing form on your own account, just go to MY FORMS > click your form > MORE > CLONE FORM

On the other hand, if it's a form that you don't own, just get the URL of the form and clone it using the steps laid out on this guide: How-to-clone-an-existing-form-from-a-URL 

Don't hesitate to get back to us if you need further help.