This just happened to me tonight. I have handcent installed and use that as my only text messaging app. I sent a text to my friend Lauren, after the spinning throbber icon stopped, the name at the top changed from Lauren to Margaret. I sent a text to Margaret apologizing for the text saying I was texting someone else not her. Her reply was that she didn't receive any texts until my apology.
I used the SMS Backup+ app to back up the messages so I could see in my Outlook just who I messaged and sure enough, the first text to Margaret was my apology. For Lauren, there are two messages - the first one that my handcent app says I sent to Margaret, and the second one is the one I copied from the Margaret window to Lauren and sent.
I already sent an email to Handcent bug reports and I was just curious if this has happened to anyone else.
I am not rooted, I am running the latest stock operating system 2.3.4 GRJ22.