Sorry folks, but it really isn't the carrier in my case
I cycled between my BB 9900 / Lumia 920 with my micro sim all the time. Same carrier, same sim. And both phones get a full 5 bars, both on H+/3G (not LTE which has crap coverage).
Blackberry never sent a message twice
Lumia sent messages twice very, VERY frequently.
I have no idea what it is. I also cycled through three phones (as i had other FCC and body creaking problems) and all of them did the same thing. Pretty annoying when you're typing a larger text and your recipient receives a freaking essay! :eck: