Welcome to Android Central! Is "Messenger" the stock Samsung app, or Facebook Messenger? Does your duplicate text also disappear after a short time, like the OP mentioned?
I have the Note 9 and everytime I send a message, I end up with a duplucation of the text. It only appears on my screen, my receipriants do not get 2 of the same message. Is there a way to stop it, it's doing my head in. Only had phone for 4 days.
I'm having a similar problem also with the Note 9 but my duplicates are texts I'm receiving, not all of them but various messages are coming in twice. Any suggestions?