Let me guess, its a tmobile number.. i have friends with a tmobile account and sometimes i get 2 of the same messages or they get 2 of the same messages from me..
Not sure this is Droid related, as I have had similar reports from non-Vz friends when texting from my EnV in the past couple of months. Both times a power cycle of the phone corrected the problem. Also had 2 other friends with Verizon phones get similar reports from their non-Vz text recipients.
Being a former insider with Cingular, then ATT, I can say that this happens on their network quite a bit. ..I won't go into detail but I'll just say there is room for improvement there...