Have occasionally had a sms sent only to have error messages saying not sent, in the time that it takes to resend, the recipient has replied to the apparently unsent message. Not a major problem but has only happened under Android 10 beta.
Have sent error feedback
Hi @Graykenhill
Thanks for sharing an error report with us!
To be honest, it does seem like a big problem, since no one wants to pay more for messages that he needn't have sent 
