From the Project Page
When you create a new design, you can choose to copy any preexisting design from the drop-down menu. This will automatically populate the full design, including module layouts and electrical assumptions. This is useful for analyzing slight adjustments to a given design (e.g. row spacing, tilt, DC-to-AC ratio, etc).
From Within a Design
When you are in a design you can create a duplicate version without ever leaving the design. This is especially useful if you have a variety of design alternatives you want to evaluate.