title: InvalidAuthenticityToken in Devise::SessionsController#destroy (sign out after already having signed out)
body: <p>I am using <a href="https://github.com/plataformatec/devise">Devise</a> <code>3.2.0</code> for authentication and found an issue when I do the following:</p>
  
  <ul>
  <li>tab 1: sign <strong>in</strong> to app</li>
  <li>tab 2: go to any page in the app</li>
  <li>tab 2: sign <strong>out</strong> (success)</li>
  <li>tab 1: sign <strong>out</strong> (failure - see exception below)</li>
  </ul>
  
  <p>Exception raised:</p>
  
  <blockquote>
    <p>ActionController::InvalidAuthenticityToken in Devise::SessionsController#destroy</p>
  </blockquote>
  
  <p>In the development log I see:</p>
  
  <blockquote>
    <p>Can't verify CSRF token authenticity</p>
  </blockquote>
  
  <p>And the top three lines of the stack trace are:</p>
  
  <pre><code>ActionController::InvalidAuthenticityToken (ActionController::InvalidAuthenticityToken):
    actionpack (4.0.0) lib/action_controller/metal/request_forgery_protection.rb:163:in `handle_unverified_request'
    actionpack (4.0.0) lib/action_controller/metal/request_forgery_protection.rb:170:in `handle_unverified_request'
    devise (3.2.0) lib/devise/controllers/helpers.rb:198:in `handle_unverified_request'
  </code></pre>
  
  <p>How can I ensure successive <em>sign outs</em> don't raise an exception?</p>