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This happens largely due to most websites using cookies to keep users logged in. They do this to cause fewer sign-ins, making it easier for users to use their website without getting prompted to log in. However, this can cause problems when there are multiple accounts used on that website by that user, or because multiple users use the computer, especially if someone has clicked a "keep me signed in" option at some point.
Resolving the Auto Sign-In
This should log out the currently-used account, and prompt you to log in again.
Prevention
To keep this problem from happening, there's a few things you can do:
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