You can easily duplicate a survey, whether it is open, closed or a draft.
To do this go to the "Manage" page of a particular survey and click on the wave you want to duplicate. On the right hand side of the page, it will show you the targeting you've selected, click the three dots on the top of the page (next to the status) and this will give you the option to Duplicate & Increase sample.
When you click duplicate, the duplicated survey will now appear in your drafts and you can make any changes to it before purchasing.
N.B. - you may wish to change the public or internal title of the new survey, to allow you to easily differentiate between them.