I'm not a Bungie Employee, but most likely you've checked some box which says "Remember my login" when logging into PSN's service in the past and now your browser automatically remembers and submits your credentials in an attempt to be helpful. Clear your browser's cache and you should be able to log onto either account.
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I did not click "Remember my login", yet it still logs me in automatically even after I cleared my history. And my current Account needs to be delete as it's a fake Account and somehow it got linked to my normal Facebook/Ps3 email..
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Clearing your history will do nothing. You can either sign out directly from Sony Entertainment Network's site or you can clear your cache and cookies.