"cannot redeclare class" tells you that it's trying to load the same file a second time when it's never intended to do so.
So, that means you've got something in one or more of your site files that's throwing the code into a loop.
Common causes of such would include:
- making a backup of a php file by renaming abcd.php to abcd-bak.php instead of deleting the ".php" from the end of the backup file's name
- poorly written addons
- incorrectly installed addons
- rogue activity on the site: http://www.zen-cart.com/wiki/index.p...ing_From_Hacks
- damaged files: http://www.zen-cart.com/wiki/index.p...Obscure_Issues