When on the "Assignments" sub-tab in a class, select the 3 action dots next to the "Edit" button for the assignment that you would like to duplicate.
Choose Duplicate from the dropdown menu.
You will be taken to a screen similar to the Create Assignment screen, however, all of the information for the original assignment will already be filled in. You may edit any of the information on the screen, just as you would for creating a new assignment.