You know how it goes
Pass the buck
I have two of the same phone with the same Android releases one's on AT&t network one is on another Network it's only the AT&t phone that double texts sometimes
previously it was due to issue communicating with the network, some sort of delay in the sending/receiving of data causing duplicates to be produced.
I think it's a network issue (ATT issue) but you can confirm by submitting error report to Samsung Members after seeing the issue again. Submit a ticket right away.
Please share the device's model number, software version and the samsung messages app version. thanks.