PayPal duplicate orders, incomplete orders

Zen: 1.5.7c
PHP 5.6
PayPal Express
Few if any plugins to the base Zen code.

We often get duplicate orders, say 1 in 100 orders. Orders that are a couple of seconds apart and have the same PayPal transaction ID, but unique order numbers

We also get emails from customers who are convinced they placed an order. While there is a matching Zen account, there are no orders associated with it and no charge at PayPal. We ask them to login where they will likely find their order still in the cart. We assume this is a PayPal issue. This is maybe 1 in 200 orders.

We saw this same behavior with Zen 1.3.9h and our hope was that it would go away with the upgrade to v1.5.7c. To increase the chances of this, we did not upgrade our existing files/changed files, we instead went with all new v1.5.7c files and only imported the DB from v1.3.9h. We did the upgrade around January. For the most part, orders are issue free

Anyone else see these issues with PayPal Express?

Upgrade to 1.5.7d and you'll get three or four new PayPal changes to the files.

That might help.

IF you did a new start with your 1.5.7c, I have a zip of the files that were changed in 1.5.7d on github.

BACKUP

Is there any reason to believe that these changes in 1.5.7d would resolve or improve the issues I described? Are these known issues?

I doubt seriously that they would make changes to PayPal settings that weren't needed.

Both PayPal and Square are changing this year. Square next month. The new WebPay fixes the Square and none of our 1.5.7d sites are reporting your symptoms with the new PayPal files.

Also PHP 5.6 is way out of date and no longer under support. If you are running Zen Cart 1.5.7d, you should be using PHP 7.4.