Occasionally my phone will tell me that a text message has failed to send--but the recipient will report receiving multiple copies, in one case as many as 50! This was annoying when it only double-sent the messages, but 50 duplicates is a huge problem for the recipient.
I live in an area where the cell reception isn't always great and so it's not that unusual for a message to fail to send or to have to try a little longer to send. But when the duplicates happen, it's always the same pattern: my phone reports that the message didn't send at all, but the recipient gets multiple copies. There's never any record of multiple copies on my phone. I use the default Messages app on my HTC One M8.
This only happens sporadically so I can't say exactly when it started or how long it's been happening. And it's going to be hard to tell if it's been fixed.