Ruby on Rails

Hi
   I am working with ruby on rails application.
   I can make a user to signin facebook when he click the facebook button.
        I am having a button as Add facebook button, in that, when the user click the button, he need to add one more facebook account.
        Since a user is already logged in, i cant able to show the facebook signin page, it again logging with the same account.

Help me to solve this,
    I thought of logout the previous account, and make signin for the new user.... but for logout we need previous user access token.. how can we get who is logged in presently in our system.....  Is there a query to find a person who is logged in presently in facebook

Thankx

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