You can easily copy a specific layer or the entire banner content from one banner to another by simply using your keyboard. So just select the content and copy it ( CTRL+A / CTRL+C).
Then in the same tab create a new banner and hit CTRL+V on your keyboard to paste the layers.
This is really helpful when you want to start a set from a single banner and the process is exactly the same. Just remember that in the Generator you need to use the Focus mode to paste the content.