Google Stadia actually charged me multiple times for amounts that should have been refunded
That sounds a bit odd, can you see the charges also on pay.google.com? Whenever Google is charging you for a service, you would see the transaction there.
Could also be that something is wrong or odd on your bank/creditcard statement?
Maybe it makes sense in that case to reach out first to your bank to ensure that there have been really charges happened and after that get in touch with Stadia support if you really have been charged.