iOS 15 iPad Mail Interlocked Double Square Icon?

I have the same problem - how do I eliminate the other “instances” of Apple Mail running? Or at least permanently kill these thumbnails?

For supported Apps (such as Mail), The iPadOS15 Operating System permits you to have multiple “instances” of the same programme running.

Should you have two or more “instances” of the Mail App running, tapping the illustrated icon will cause a thumbnail image of each running Mail programme on the “Shelf”; here you can simply switch between running instances by tapping the chosen thumbnail.

If you only have a single instance of Mail running, you will only see one thumbnail on the shelf.

Okay, now that I understand what it is, how can I turn it off? I only have one version of mail running (that I know of—unless you count versions running on other devices, e.g., iPhone, iPad, MacBook) yet these keep popping up when I open mail. These things are just annoyances. I don’t want to see them. Please make them stop!

As soon as I open mail on iPad I get a whole bunch of thumbnails displayed automatically. Any idea how I can stop this happening? This happens without touching the interlocked double square icon.

How do I get rid of the double interlocking square icons?

The icon is annoying, I guess getting rid of it because nobody likes it in the next update might help solve the problem or they could add an option where you can shut it off in settings.