Error message "Mage registry key already exists" e.g. after extension installation
Mage_Core_Exception: Mage registry key <...> already exists
The solution is very easy in most cases:
If you recently installed a new Magento Extension, please confirm that you followed all installation steps that are mentioned in the
README file of the module.
In all other cases especially assure that the Magento cache is cleared, e.g. by emptying the
var/cache/ directory in a Magento default installation and that the Magento compilation process has been triggered (if enabled).
If you need help with this problem, just contact me.