Hello!
I made meanwhile the following test: switched to Astra and Twenty Twenty themes. With these no double buttons! When I switched back to xStore Child theme then all was wrong again 🙂 Double buttons exist also with xStore theme.
One more observation. If I refresh my cart page, then everything is fine during the blink of an eye. Maybe this page is loading two times?
I send my ftp access in private content area.
Thank you.