Every time, when You need a new unique Product Structure, You can create it step by step absolutely manually... But, when the new Modification or/and Version of Product Structure should look like one existing, the best way to keep Your time - is to create a new Product Structure as a copy of existing one. The main reason to DUPLICATE existing Product Structure – is to use existing Components List with related details as a basis for the new Modification and/or Version of Product Structure where some required changes for Components List will be made manually.