As a follow up. I too thought it had something to do with wi-fi vs not, Bluetooth, etc. I thought it was a connection combination issue. The problem is in trying to test all those scenarios over many months, I ALWAYS ended up with the problem resurfacing
no matter which configuration I chose. And I never found any of those that I could turn on/off and instantly have the problem resolve.
Once I discovered the AT&T Backup & Sync effect on the dupe problem, and I turned it off, I've moved in and out of combinations of connections, including wired, wi-fi, etc. I have not had a single dupe in any configuration since I turned off the Backup
& Sync function.
If I turn that service back on, they start up again immediately.
Back in my software development days, that is exactly the kind of "switch" we looked/hoped for tracking down bugs. I'm hoping Microsoft can use that info to replicate it reliably and trace their way into the thing. My concern is it's a fundamental problem
with the message database on the phone and not a Your Phone app problem. I'm guessing there's an API that Microsoft is using to access the message data, and they may be using it entirely correctly. If more than one service is using it, maybe it's the code
behind the API that's failing, and not Microsoft's code.
Jeff A.