Maybe a bug in your own code, but not a bug in Zen Cart in general. (Moving post out of the bug reports area)

"cannot redeclare class" only happens when a PHP file containing the class specified gets loaded a second time.
And that usually only happens when you have extra copies of files left on your server such as shopping_cart_orig.php alongside shopping_cart.php etc, or if you have accidentally put the same class file in another folder whose contents are automatically processed such as extra_functions or extra_datafiles etc.

Find out the pattern behind "it only happens sometimes", and then get rid of the duplicate loading that's happening.