OK, I've done a bit more research into this issue and there is a setting right in the RCS settings in Messages that deals with these duplicates. I do not believe this is a real fix, but a band-aid for carrier network switching issues.
I will outline my issue again and the possible solution I've found:
PROBLEM: intermittent duplicate text messages (SMS) and chat messages (RCS) when communicating between Android Messages on Rogers network with both Messages apps set and registered to use Chat features / RCS
SOLUTION: Messages--> Settings--> Advanced--> Chat Features--> Resend Messages If Undelivered--> Ask before resending
POSSIBLE CAUSE: [recipient who is connected to Wifi but is not 'solidly connected' to the Rogers network]: when the RCS chat message is sent to this recipient and fails, the Jibe server asks the Rogers network to have the message resent as SMS text message (which it does and delivers it successfully over whatever cellular data pathway), but in the meantime the message is also resent as an Internet RCS chat message and arrives later (duplicate)
OK, I've done some limited testing with this 'resend messages if undelivered' setting, and it fixed my duplicate messages.
I sent the recipient message at 6:04 this morning.
At 9:22 the phone was picked up for the first time in the day, and the message was read and the recipient had no duplicates. Sent and received as RCS with no SMS re-try.
Later today the message in my phone's Messages still says this below it: "Not delivered yet. Tap for options." Even though the message was delivered and read at 9:22. Right now if I select the message it pops up a dialogue that states: "Recipient is offline" and a button to send as SMS/MMS.
Later on another test message was sent and the recipient phone was dormant(?) again, there was a delay on delivering the message but this time the button to resend the message was used. Duplicates showed up on the recipient's Messages. One RCS and one SMS.
My wife uses a pixel 3 XL (software up to date) and she keeps getting text messages of one's she already gotten days before. It's happened with different people she texts (1 person was on iPhone Xr and the other on a galaxy s20). She uses Android messages app.
That certainly sounds like an irritating issue to experience for sure! Receiving multiple messages from senders like this can happen occasionally if the sender has poor connectivity at the time of sending, the message will attempt to send at a later time, though this is usually right away it can be delayed. If it's happening with sent messages though I would definitely recommend reaching out to support at your convenience. Definitely let us know if you have any further questions.