Update! I have officially figured out what is causing the duplicate text messages after updating to the latest messages app.
In the newest releases, like the ones that came on the S9 and S9+, samsung integrated the message service app into the messages app. This, for some, is causing duplicate text messages when receiving messages from people who are on the same service provider who's devices also have advanced messaging with chat features.
To fix the problem of duplicate texts, you will need to purchase and install package disabler pro ($2.50 - don't be cheap...) from the play store and disable message service app and reboot.
https://play.google.com/store/apps/d...pdp.singleplay
That's all there is to it enjoy no longer getting duplicate text messages just as I am.
Happy Tuesday!