The main way that what you describe can happen is in the following case:
* You have an account for user A on your site.
* You open 2 separate tabs in your browser. Both for your home page.
* In the first tab, you login with user A
* The second tab still shows no user has been logged in, because it was opened before A logged in. In this second tab, if you try to authenticate with Facebook or Google, JFBConnect will think you're trying to update user A's account to be associated with the Facebook/Google account you're authenticating with
There's not a way to fix this as the browser is doing what it's supposed to by sending the proper cookies signaling the same browser session. Joomla is doing what it's doing and letting JFBConnect know that a user has already authenticated during this session. JFBConnect is doing what it thinks is right in associating the social login with the existing Joomla account that is logged in.
I hope that helps explain. The only way we've *ever* heard of this happening is the above scenario. It only happens when you're using the same browser, on the same computer, to login to multiple accounts in different ways. It's not something that would normally happen in the real world and we haven't heard of any real-world scenarios of this being a problem.
However, please test and let us know if you think there's some other course of action that causes the accounts to be incorrectly logged into, as you note above and we'll gladly help investigate further however we can.
THanks,
Alex