It's happened to me a few times, but you may have hit the send button twice, perhaps because of your device. I mean, you're not sure if you sent the message and you hit the send button again.
Sometimes this happens when clicking the back button after posting something it submits again I notice not all the time though. Also the other thing is if you click the button again after thinking it didn't go through it may have so wait a little longer next time.
it definitely happens when you backpage after submitting, figured that out the hard way, then tested it to make sure. Idk if happens any other way however.