I have done my medical back in 2001 but got that letter which basically said they need a medical but at the bottom, they said if already done, to provide evidence of such being done. I consulted the attorney and attorney said that I should try to visit the SAME doctor and get another copy of the file (in the SEALED envelope). That I did and resubmitted...just in case they lost my original one.
Also VERY important (As per my attorney):
The letter said to include an evidence that the doctor is so-called approved physician. This is rather strange requirement, since 'ordinary' physicians will not issue these medical forms in the first place. However, just print the list of doctors from your area form USCIS website, highlight your doctor and include this paper in the mail you will send.