Wirelessly posted (My IV to the Net: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry7250/4.0.2 Profile/MIDP-2.0 Configuration/CLDC-1.1)
Hey redbeard911, this has happened to me once or twice and I am 95% sure it is because once I submit a post I hit the Back button, which brings you back to the "You message has been added" screen, then you go foward from there which (re)submits your post again.
I'll try it with this post to confirm.
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That is it, 100% sure now.

This usually happens to me when I holster my BB after making a post, time passes I pull the BB out again and almost automatically hit the Back button. We just have to remember to use the bread crumb trail (of links) at the top of each screen to return to a forum or thread, instead of the Back button.
Hope that helps.