You might think: "Why would I need two banners in the same format when I can simply use the same format twice?".
But if you need to e.g. create a wallpaper you might need a left and right banner in the same format but with a mirrored design.
Follow these steps:
Open up your template in Studio
Click on formats in the upper left corner
Here you have listed all of your formats
Three dots are shown behind each format. Click here
Now choose "Duplicate format"
Happy building! 👏