title: Magento - Mage registry key "_resource_helper/core" already exists
body: <p>I'm migrating a magento 1.6.1 installation on my staging server, but I'm still getting the error <code>Mage registry key "_resource_helper/core" already exists</code>.</p>
  
  <p>I migrated the db and the whole source code. Then I changed <code>local.xml</code> database settings.</p>
  
  <p>I already ran <code>rm -rf var/cache/* var/session/*</code> and restarted Apache2 but can't get it working.</p>
  
  <p>What should I do?</p>