I had the same situation 2 years ago, when my parents were been requested to do the the immigration medical exam (now my in-law is waiting).
Two years ago, the CIC did not automatically know we have just done the medical exam. We have been requested to do the immigration medical exam after 2 months we finish the medical exam for visitor extension.
After receiving their email, we sent letter, email, and fax to Ottawa office to request them to use the medical for visotor extension we have just done to replace the request for immigration. I have fight for almost two months!! With the help of my medical exam doctor office and the MP, I was successful to use the medical requests for visitor extension to replace the immigration one in the end.
In the meantimes, I got to know other 4-5 guys online which have the same situation. All of them informed CIC by writing but no response as well. What I learnt is that almost all of them re-did the medicals as there were no responses from the Immigration Office in Ottawa at all, one even waitting for 4 months.
If it happens again, I will not wait for their response anymore. Instead, I will just simply do the medical again. It is not worth and the writting was driving me carzy.
I do not know if the CIC changed since then, but I am telling you what was happened. You may also searched this forum as I had many post at that time.