Open the projects browser screen, where you projects are displayed as icons. Move your cursor over the project icon and that will cause a little circle with dots to appear to the right of the project title. Click on the circle and then click on Duplicate Project in the pop up menu that appears. The will create a duplicate of the project.
If still no luck, try deleting preferences by opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can duplicste a project.
—- Rich