SMSes have confirmed delivery. If the SMSC does not get the confirmation (on lower protocol layer, nothing to with delivery reports) it will send the SMS again. So duplicate delivery is possible. Because there is no unique identfier, the phone cannot eliminate such duplicate.
Delivery reports are best effort. The phone just sends them, but if the reception fails they are lost.
I live in a country with excellent mobile coverage, worked in the GSM industry already in the 1990s and have seen double delivery or missing report only less than a handfull of times in 25 years.
When I travel abroad I'm shocked how bad the network is in some industry countries. Many bizarre things happen as the parent writes, but not only related to SMS.
Edit: Above explanation is shortened. Of course there are always at least 2 hops: Sender to SMSC, SMSC recipient. I guess both hops are either confirmed or best effort. But it's been 28 years since my GSM training...
Delivery reports are best effort. The phone just sends them, but if the reception fails they are lost.
I live in a country with excellent mobile coverage, worked in the GSM industry already in the 1990s and have seen double delivery or missing report only less than a handfull of times in 25 years.
When I travel abroad I'm shocked how bad the network is in some industry countries. Many bizarre things happen as the parent writes, but not only related to SMS.
Edit: Above explanation is shortened. Of course there are always at least 2 hops: Sender to SMSC, SMSC recipient. I guess both hops are either confirmed or best effort. But it's been 28 years since my GSM training...