The duplicate option creates a new document for each document you have chosen to duplicate. If you have selected a single document out of a set (and are previewing it) then pressing duplicate will prompt the page to check and make sure you want to duplicate that single document, and go forward if you confirm. If you are viewing the set of AHAs/Plans instead, the page will ask if you want to create a new set of documents based off of the set you are viewing. If you are duplicating AHAs, you will be taken to Select a Template. If you are duplicating plans, you will get a notice that makes sure you understand what you are doing before progressing to Plans.
NOTE: duplicating documents creates new documents and counts towards your overall total.