Why do mobile phones often automatically resend the same message twice?

This happens when the network signal is not good.

Because there is a process of sending short messages, the signal is interrupted during the process of sending short messages, and the feedback from the mobile phone is that the short messages have not been sent and should be sent again, but in fact, it is also possible that the short messages have been sent and the information center has received all the information.

It has nothing to do with mobile phone settings. Where I used to live, the signal was bad. My mobile phones there are all like this.