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.
I've seen it before with users switching from iPhones back and forward. It's due to Apple's bad way of handling iMessage. Since your number gets stuck with it in their servers. Try to remove the number from the Apple ID or try to un-attach it.
My mom has a 635 and this doesn't happen with her phone, though this happens to virtually every model of phone (not just WP, but Samsung, HTC etc) when leaving Iphone behind if their services aren't fully deactivated, as jwinch2 suggested.