Yes, duplicating then deleting portions is the way to go, as described by elikness.
While in Project Library view (not in the editing view), click on the project name then select File > Duplicate Project from iMovie's Menu. Alternatively, right-click (or Control-click) on the project name in the Project Library, then select Duplicate Project from the pop-up menu.
Create as many duplicates as you need. Each one will be given the same name as the original project, but with a number appended, such as "project name 1", "project name 2" and so forth. The names can be changed if preferred.