A friend of mine once found she'd somehow managed to sign up for Prime in America despite already being a Prime member in the UK. She only noticed 6 months later. Got her money back though. To this day she still can't work out how it happened.
https://us.amazon.com/gp/help/customer/display.html?nodeId=2... Change your Amazon Account Country
Important: If you create a new Amazon account instead of transferring the existing account, purchases won't be shared between the accounts. You’ll have separate login credentials for each account.