Perhaps I’ve found it. I also added Site Kit by Google and it also hooks into Google Analytics along with Search Console, Ad Sense, Page Speed Insights and Tag Manager. So I really like this one-place-to-look approach. Deactivating Monster Insights proved to be the solution because I no longer have duplicates according to Chrome Debug.
Therefore it is the case that it is a choice between Google Analytics by Monster Insights and pure Google Analytics.
I do see some benefits in the WooCommerce <==> Monster Insights partnership but the extra value of Monster Insights seems to me to be the paid additions (e.g. eCommerce) that I’m not prepared to pay for at this time.
I think for the time being I’ll remove the Monster Insights version and end this thread.