You aren't alone. I get duplicate text messages from my wife from time to time. She is also a WP user. It started when I switched to iPhone and then switched back to WP with a 1520.3. Sometimes I only get one duplicate, but other times I get the same message seven or eight times, even after I have already responded. Its fairly rare, but it is certainly annoying when it happens.
I have no idea what is causing it, or how to make it stop.