Yes, very normal. When my husband was approved, he got an email (via our consultant) about being approved, I never received the actual AIP letter from them. I called a few weeks after he received his approval letter and they emailed me another AIP approval. Granted, I lost a few weeks, because they didn't back date it (I needed it for OHIP) but it wasn't a big deal. If you must have it, then call CIC and ask them to reissue the email if they would be so kind, but if you don't require it for some other kind of paperwork, it is not really a necessity.
Good luck.