Mage registry key "_singleton/customer/session" already exists

#0 /home/uniscom/public_html/app/Mage.php(223): Mage::throwException('Mage registry k...')
#1 /home/uniscom/public_html/app/Mage.php(477): Mage::register('_singleton/cust...', false)
#2 /home/uniscom/public_html/app/code/core/Mage/Customer/Model/Observer.php(98): Mage::getSingleton('customer/sessio...')
#3 /home/uniscom/public_html/app/code/core/Mage/Core/Model/App.php(1338): Mage_Customer_Model_Observer->beforeLoadLayout(Object(Varien_Event_Observer))
#4 /home/uniscom/public_html/app/code/core/Mage/Core/Model/App.php(1317): Mage_Core_Model_App->_callObserverMethod(Object(Mage_Customer_Model_Observer), 'beforeLoadLayou...', Object(Varien_Event_Observer))
#5 /home/uniscom/public_html/app/Mage.php(448): Mage_Core_Model_App->dispatchEvent('controller_acti...', Array)
#6 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(302): Mage::dispatchEvent('controller_acti...', Array)
#7 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(259): Mage_Core_Controller_Varien_Action->loadLayoutUpdates()
#8 /home/uniscom/public_html/app/code/local/Bit/Exibition/controllers/IndexController.php(6): Mage_Core_Controller_Varien_Action->loadLayout()
#9 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Bit_Exibition_IndexController->indexAction()
#10 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('index')
#11 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#12 /home/uniscom/public_html/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#13 /home/uniscom/public_html/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#14 /home/uniscom/public_html/index.php(89): Mage::run('', 'store')
#15 {main}