The duplicate option allows you to copy the segment so you can refine the segment without adjusting the original segment.
To duplicate, click Audience > Segments > then click on the triple-dot menu to the right of the segment to duplicate. Click the duplicate option in the drop-down menu.
The duplicated segment will then open up on your screen. Further refine your dataset by selecting different criteria for your segment. You can also rename your segment by clicking on "Rename Segment" in the triple-dot menu. Click save to save your new duplicated segment when you are finished.
Alternatively, you can click into a segment and duplicate the segment by clicking on the arrow menu on the upper-right-hand corner and select "Duplicate" from the drop-down menu.