Zen Cart: 1.5.5f
PHP: 7.1.29
Payment Modules : Paypal Express & Heartland

I installed the TaxCloud code and I've been having an intermittent issue with duplicate orders.

The issue has occurred with both payment modules and for states that collect sales tax and states that don't.

No PHP errors.

I'm not able to recreate a double order myself.

It appears as though the users arrive at the checkout confirmation and start circling back to the site.

I disable the paypal express button because I noticed those users tend to leave the confirm page from time to time to edit their shipping information.

Since I have done that I'm not seeing any duplicates with paypal but that could just be based on user behavior going forward I guess.

Also, the orders can be as far apart as a couple minutes in time (time stamp). They aren't 4 to 5 seconds apart.

I noticed while the customer is getting charged for both orders, zen cart is not posting the data back from the payment processor (authorization code, etc)

I don't have any redirect plugins and the cart was trouble free before the taxcloud installation.

Over 90% of the time, it works great.

I'm a hack at best when it comes to code.

Is there something I should be looking at specifically.