Drupal broken after upgrade to CiviCRM 4.6.16
I don't think that error is related to your CiviCRM upgrade, at least not directly. That error message is telling you that a duplicate function exists in your custom theme's drupal6_theme_methods.php file and garland's template.php file. Neither of those files are in the CiviCRM directory, which is the only code you replaced during the upgrade, right?