Hi, denise,
iMovie automatically, as you are working, saves your movie to the projects browser screen (where your projects are displayed as icons). Click on the Project back button to get to the Projects browser.
After clicking on the Projects back button the Projects browser screen will open up. You will see your projects displayed as icons.
Move your cursor over the title of the project and that will reveal a little circle with dots in it to the right of the title. Click on the circle and you will get a drop down menu where you can choose to duplicate your project.
When you click on Duplicate Project, your duplicate project will appear in the Projects browser screen. From there you can open it and edit it. Your original project will remain intact.
-- Rich