In Session, with the help of SessionState, there is code to keep track of whether a logout message has been sent. In the disconnect method this flag is normally cleared. However, if the method returns early due to the session being already disconnected, the flag is not cleared.

So the flag remains set, and this later triggers another logout (after the session has really been re-established). (And the reason for that logout is logged incorrectly – see bug posted minutes ago.)