Mage registry key "_singleton/sagepaysuite/observer" already exists

Trace:
#0 /domains/consortium.co.uk/___deploy/releases/4/app/Mage.php(226): Mage::throwException('Mage registry k...')
#1 /domains/consortium.co.uk/___deploy/releases/4/app/Mage.php(480): Mage::register('_singleton/sage...', false)
#2 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Model/App.php(1336): Mage::getSingleton('sagepaysuite/ob...')
#3 /domains/consortium.co.uk/___deploy/releases/4/app/Mage.php(451): Mage_Core_Model_App->dispatchEvent('controller_acti...', Array)
#4 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Controller/Varien/Action.php(319): Mage::dispatchEvent('controller_acti...', Array)
#5 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Controller/Varien/Action.php(264): Mage_Core_Controller_Varien_Action->generateLayoutXml()
#6 /domains/consortium.co.uk/___deploy/releases/4/app/code/community/EcomDev/Varnish/controllers/EsiController.php(96): Mage_Core_Controller_Varien_Action->loadLayout(Array)
#7 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Controller/Varien/Action.php(418): EcomDev_Varnish_EsiController->handleAction()
#8 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('handle')
#9 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#10 /domains/consortium.co.uk/___deploy/releases/4/app/code/core/Mage/Core/Model/App.php(365): Mage_Core_Controller_Varien_Front->dispatch()
#11 /domains/consortium.co.uk/___deploy/releases/4/app/Mage.php(686): Mage_Core_Model_App->run(Array)
#12 /domains/consortium.co.uk/___deploy/releases/4/index.php(101): Mage::run('', '')
#13 {main}