I had this problem on my X recently, it was due to a theme I was using. There was an extra file in the System folder, Mms.apk that was causing the duplicate messages. I used Root Explorer to rename the file to Mms.bak to solve the problem.
Don't know if your problem is due to a similar situation.
I do have a theme on my phone I was able to find Mms..apk under system/app so I renamed it to Mms.apk. Is this the area in which you found Mms.apk on your Droid X?
Thanks for the help!
No luck with that it deleted my whole text messaging anyone with ideas?