Sorry--don't have a lot of time to scroll comments...but if any of you got the request for a police check and you already sent it with your app....note that this happened last summer as well. You'll probably notice the letter states "IF you have not yet submitted the following documents" As well as mentioning to disregard the letter if you have already submitted the required documents.
A couple phone agents confirmed this is generic email. To be safe, you can just upload (via Case Specific Enquiry) a copy of your police check, the letter you received with a note stating the original was already sent --with your app/or however it was submitted.
Thanks CIC for confusing 100's of applicants AGAIN!!
Remember:
If you need a police certificate from a country or territory and:
You are currently living there, or received the police certificate before leaving, the police certificate must be issued within six months before you apply.
You have lived there in the past, the police certificate must be issued after you last lived in that country or territory.
http://www.cic.gc.ca/english/information/security/police-cert/intro.asp