XF 1.2 Server Errors Help

Code:
Zend_Db_Statement_Mysqli_Exception: Mysqli statement execute error : Duplicate entry '8f1866d22d55497c6c331588aad2a49f' for key 'PRIMARY' - library/Zend/Db/Statement/Mysqli.php:214
Generated By: Unknown Account, Yesterday at 7:30 PM
Stack Trace
#0 /usr/www/joinsng/public/library/Zend/Db/Statement.php(297): Zend_Db_Statement_Mysqli->_execute(Array)
#1 /usr/www/joinsng/public/library/Zend/Db/Adapter/Abstract.php(479): Zend_Db_Statement->execute(Array)
#2 /usr/www/joinsng/public/library/Zend/Db/Adapter/Abstract.php(574): Zend_Db_Adapter_Abstract->query('INSERT INTO `xf...', Array)
#3 /usr/www/joinsng/public/library/XenForo/Session.php(765): Zend_Db_Adapter_Abstract->insert('xf_session', Array)
#4 /usr/www/joinsng/public/library/XenForo/Session.php(506): XenForo_Session->saveSessionToSource('8f1866d22d55497...', false)
#5 /usr/www/joinsng/public/library/XenForo/Controller.php(412): XenForo_Session->save()
#6 /usr/www/joinsng/public/library/XenForo/Controller.php(356): XenForo_Controller->updateSession(Object(XenForo_ControllerResponse_View), 'XenForo_Control...', 'Index')
#7 /usr/www/joinsng/public/library/XenForo/FrontController.php(354): XenForo_Controller->postDispatch(Object(XenForo_ControllerResponse_View), 'XenForo_Control...', 'Index')
#8 /usr/www/joinsng/public/library/XenForo/FrontController.php(134): XenForo_FrontController->dispatch(Object(XenForo_RouteMatch))
#9 /usr/www/joinsng/public/index.php(13): XenForo_FrontController->run()
#10 {main}


Code:
Request State
array(3) {
  ["url"] => string(56) "http://joinsng.com/threads/sng-global-server-rules.1011/"
  ["_GET"] => array(0) {
  }
  ["_POST"] => array(0) {
  }
}


What exactly is causing this, and how can I fix it?
 
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