Duplicating elements allows you to quickly build out multiple versions of the same customization styles you have already done and that need just slight changes, saving you of creating new elements from scratch.
How to duplicate Pages or Graphics
A) For Pages: Go to Engage > Pages
B) For Graphics: Go to Engage > Graphics
(1) Click on the 3 dots over the element you want to duplicate
You should see a drop down menu appear.
(2) Select the 
Duplicate option
A copy of your elements should appear on the same Experience.
Note
For Pages, elements keep the same IDs in the original page and the duplicated, that way if you have added any CSS styling, you will see those styles on the duplicated Page as well.
For Graphics, the elements keep the same customization styles and the data sources already connected in the original will be also connected in the duplicated Graphics set.