a quote by DrByte.

"cannot redeclare class" means that somewhere in your program code that class is being loaded twice.
You'll have to hunt down where it's loaded and stop it from loading a second time.
Perhaps you have duplicate or backup copies of files that are getting automatically loaded ?
have you renamed any core file ? have backed up any file that is still on your sever ? zencart autoloads many files , if you have renamed a file that still contains .php extension ,might be loading as well causing this error.