I have experienced this several times now. A customer messages me. I'm away from my desktop or tablet but can answer their question quickly through the Etsy app. I create my reply and click send. it says it failed to send. I resend. It says it failed again. However, when I get to my desktop it shows as being sent twice! The first time I did this I didn't know it was actually sending and finally gave up after I unknowingly sent my customer SIX product pictures!
Am I the only one experiencing this? If so, I'll uninstall the app and reinstall it. If I'm not, then Etsy needs to fix this. It's been an ongoing problem for me since November.