I've never taken an animal physiology class but think it would be fine! The content is very similar and taking both will involve a huge amount of overlap. You're better off filling that extra spot with another MCAT-related course, a class that involves a lot of critical reading, or even a relaxing / "easy credit" class that will keep you from becoming too exhausted. This is especially true if you plan on taking the MCAT soon. The new MCAT has also decreased its focus on physiology (just look at the official AAMC sample test in comparison to any bio section from an old AAMC).
All things aside, for me, human physiology was by far the most helpful class that I ever took for the (old) MCAT, but even back then, it had little to do with the actual content. Instead, it "teaches you to think" about the body in a logical, sometimes multi-step way - as opposed to intro bio or even molecular bio / biochem where there's a heavy focus on memorization. Of course, quite a bit of that is up to the professor. If you take human physiology that's taught or tested poorly, it won't be nearly as helpful as animal physiology that's taught well. And just for fun, guess what the second most helpful class I took was? Russian literature. I'm serious - nothing prepares someone for the MCAT like the ability to reason and read critically, as long as you have a basic science background and study thoroughly.
