Mage registry key "current_category" already exists

#0 /usr/local/projekte/taschenstore/html/app/Mage.php(192): Mage::throwException('Mage registry k...')
#1 /usr/local/projekte/taschenstore/html/app/code/local/Comvos/FallbackCategory/Model/Observer.php(41): Mage::register('current_categor...', Object(Mage_Catalog_Model_Category))
#2 /usr/local/projekte/taschenstore/html/app/code/local/Mage/Core/Model/App.php(1230): Comvos_FallbackCategory_Model_Observer->onCatalogProductView(Object(Varien_Event_Observer))
#3 /usr/local/projekte/taschenstore/html/app/code/local/Mage/Core/Model/App.php(1211): Mage_Core_Model_App->_callObserverMethod(Object(Comvos_FallbackCategory_Model_Observer), 'onCatalogProduc...', Object(Varien_Event_Observer))
#4 /usr/local/projekte/taschenstore/html/app/Mage.php(416): Mage_Core_Model_App->dispatchEvent('catalog_control...', Array)
#5 /usr/local/projekte/taschenstore/html/app/code/core/Mage/Catalog/controllers/ProductController.php(139): Mage::dispatchEvent('catalog_control...', Array)
#6 /usr/local/projekte/taschenstore/html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Catalog_ProductController->viewAction()
#7 /usr/local/projekte/taschenstore/html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('view')
#8 /usr/local/projekte/taschenstore/html/app/code/core/Mage/Core/Controller/Varien/Front.php(177): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#9 /usr/local/projekte/taschenstore/html/app/code/local/Mage/Core/Model/App.php(305): Mage_Core_Controller_Varien_Front->dispatch()
#10 /usr/local/projekte/taschenstore/html/app/Mage.php(596): Mage_Core_Model_App->run(Array)
#11 /usr/local/projekte/taschenstore/html/index.php(80): Mage::run('', 'store')
#12 {main}