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/Captcha/Model/Zend.php(194): Mage::getSingleton('customer/sessio...')
#3 /home/uniscom/public_html/app/code/core/Mage/Captcha/Model/Zend.php(119): Mage_Captcha_Model_Zend->_isUserAuth()
#4 /home/uniscom/public_html/app/code/local/Bit/Powerbuyer/controllers/AccountController.php(152): Mage_Captcha_Model_Zend->isRequired()
#5 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Bit_Powerbuyer_AccountController->requestAction()
#6 /home/uniscom/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('request')
#7 /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))
#8 /home/uniscom/public_html/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#9 /home/uniscom/public_html/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#10 /home/uniscom/public_html/index.php(89): Mage::run('', 'store')
#11 {main}