There are a few different ways that you can duplicate a video in iMovie. One way is to simply duplicate the project that the video is in. To do this, open the project that the video is in and click on the "File" menu. Then, click on the "Duplicate Project" option. This will create a copy of the entire project, including the video that you want to duplicate.
Another way to duplicate a video in iMovie is to export the video and then import it back into iMovie. To do this, select the video that you want to duplicate and click on the "Share" menu. Then, click on the "Export Movie" option. Choose a location to save the exported video and then click on the "Export" button. Once the export is complete, open iMovie and click on the "File" menu. Then, click on the "Import Movies" option. Find the video that you just exported and click on the "Import" button. This will import the video back into iMovie and will create a duplicate of the original video.