Hi gcweb,
Is the site owner using the same PayPal account on more than 1 website, by chance? Sounds like you could be running into an issue where the same invoice numbers are getting used which PayPal sees as a duplicate. Also, is this happening with Express Checkout or PayFlow?
I’ve already addressed the duplicate issue from multiple websites being used with the same PayPal in my dev branch on the GitHub repo. If you download that and upload it, overwriting your current plugin files, then you’ll see a new option available in the settings panel for an invoice ID prefix. you can set the prefix to different values on the different sites you may be using, and that would keep them from ever matching and eliminate the duplicate issue.
Of course, that change will be included in the 1.1.6 update when it’s released, so you’d just be getting a little bit of a head start on that.
All of that said, I would definitely recommend getting WC and PayPal for WC updated to the latest versions (which you would be on if you use the dev branch I linked you to). Of course, it’s always recommended that you backup your site files and database prior to running updates.
Hope that helps! Let me know otherwise.