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Join Date: Mar 2012
Location: Chicagoland (Western Suburbs)
I had it happen to me, and (at least in my case) I think I've narrowed down the potential cause.
I wrote a small app to send a specific text to a specific number. It works - well, ... kind of. After sending the first text, the SMS system appears to freeze, and won't allow any subsequent messages to be sent from any SMS app - not just the one I wrote - until I reboot the phone. When I do, the clogged messages are released. This would include any re-sends I may have attempted, resulting in dupes.
So my semi-literate guess is that it has to do with an incompatible SMS app, which probably locks the outbound socket for a while.
......then again, I may be way off base.