This is a difficult one to nail down for me too. It used to happen on my Bionic, never before my Bionic, but it was only some times. Likewise with my GS3. It happens in 3G, 4G, WiFi. I thought for a while it was only iPhone people too, but it isn't, my brother is on a Bionic right now and he double hits me constantly. My mother's Blackberry does too. Not always though for either of them. Some iphones seem to do it more than others, but it is very rare that my dad (iPhone user) hits me with a double text.
This one, the best guess I have put together is that it happens while switching towers for either them (which is a random guess) as it might just hit a tower, upload or download, but not be able to also hit back with the "I uploaded this already" or "downloaded already" signal that stops it from sending again or receiving again.
HOWEVER, if you believe that one we can talk about the bridge I have for sale in Arizona. Poor Arizona transportation folks, there probably isn't a single engineer in Arizona that can actually propose and sell a bridge there because everyone thinks they are getting a raw deal...