I have found further information on this problem.
When I converted the first site it worked fine, when I changed the second over to PayPal Express I was still getting intermittent duplicate and triplicate invoices.
After thinking about it the main difference between the 2 sites what the first had the changes MC had recommended:
"Now, mind you, there is a second patch that will need to be applied if you are transitioning to paypal express on ZC 1.5.7 (and possibly even so for ZC 1.5.6 primarily depending on mySQL settings). This patch is necessary because if the "right" database settings are in place, an attempted purchase will cause a similar issue as recently described. (something I investigated again to see if it was a cause/factor), it's the minorest of changes, but can make a difference.
Discussion: https://github.com/zencart/zencart/pull/3692
Files/changes: https://github.com/zencart/zencart/pull/3692/files "
When I changed these on the second site things settled down. I have received several PayPal order with no problem.
So it appears you need both to resolve the issue.
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