Samsung s20+ with AT&T as well, I've tried doing every single troubleshooting step (force quit messaging app, clear cache partition, clear cache and data of messaging app, reset network settings, turn off advanced messaging) and nothing has done the trick. Every time I send a message to anyone, either they get two of the same message and it shows as one being sent for me, or it shows as the same message getting sent twice on my end as well as on theirs. It's honestly a bit annoying and not sure what else to try at this point - the issue is definitely relatively new since one of the last updates. Hopefully, someone is able to figure out the fix/it somehow miraculously fixes itself!!!
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