Hey @ScottS21,
Thanks for posting your results. Over the weekend, I found something that seems to be significant: WiFi.
When I enabled the mobile data option, I had my phone's WiFi turned off (which was just to be sure that I
was, in fact, using mobile data). And, boom, no dupes. Yay!
Two days ago, I had one duplicate appear, which really vexed me. It vexed me until I realized that I'd kept my phone's WiFi off and --
at the time I had the duplicate -- I'd enabled my phone's WiFi. I immediately turned it off, and haven't had any dupes, since.
As a summary:
(Since 03/17/2020) With the app set to "Sync over mobile data", I've had exactly one duplicate message sent, which was sent while I had my phone's WiFi enabled; with my phone's WiFi off, I haven't had any duplicates.
Best of luck to all -- stay safe!
Dave