To duplicate a Rundown, press the three dots in the top right corner and "Duplicate".
Give your new Rundown a title. It will automatically be called the name of your existing Rundown and then "(copy)" until you change it. Click the checkbox next to "Duplicate Rows" if you also want to copy every row from your Rundown. If you do not duplicate rows, the columns and the segments will still be copied with no rows.
Hit "Duplicate & Open" to create your new Rundown and start working on it.