Dental schools will probably ask why you did that instead of bettering your gpa with more classes. Makes it look like you don't know how to manage your time well. Don't focus on making your strengths stronger, focus on making your weaknesses strong. Dental schools are looking for the whole package, after all.
Interesting to see the responses here. I'm also retaking with 22AA and 20PAT. My PAT was fairly low for the school I am applying to (UBC) and people with 1 score higher on the PAT (with same GPA as me) received an interview whereas I did not, so that is my reason for retaking. I don't see it too much as a waste of time, it's mostly review and polishing up the areas where you're weak-- and less costly then spending another year of undergrad with $400 per course to boost your GPA. But Canadian schools are a bit different since they only care about your numbers pre-interview (don't ask for EC's, recs) so I do think the responses here probably resonate some truth, but I still wouldn't go far as to say it's a complete waste of time.