have you tried placing any test orders? in my experience, duplicate orders are related to problems with sending the order email.

the sequence goes something like this:

- customer hits button to place order
- timeout issue occurs due to email problem
- customer sees error on screen, and browser asks if customer wants to resubmit form.
- customer proceeds to do so; and second order is placed.

ZC does NOT clear the cart after the order gets created: that happens after the email gets sent.

see:

https://www.zen-cart.com/showthread....11#post1312911
https://www.zen-cart.com/showthread....41#post1311441
https://www.zen-cart.com/showthread....45#post1302345

good luck!