"Most Loved" Doesn't Dedupe by User

[MaggieFero]

Describe the bug
On the instance/about page, there's a "Most Loved" book listed, which takes into account the total number of ratings but not the total number of users. This currently means that if a single user has many ratings of a book (such as due to an import error), they can accidentally outweigh the rest of a small server.

To Reproduce
Steps to reproduce the behavior:

1. Go to the /about page of any small instance (like https://bookwyrm.social/about, but it's not as visible there because so many people use that one)
2. Click on the title of the book listed as the server"'s most beloved book,".
3. Scroll down to see who left the ratings
4. See that it's often one person with quite a few ratings.

Expected behavior
This slot should reflect something more than one person has reviewed.

Screenshots
If applicable, add screenshots to help explain your problem.

Instance
I have already confirmed that this affects boundcovers.com (7 ratings by one person) and comelibros.club (15 ratings by one person).

Additional context
It's most visible when an instance is small but not single-user.

---

This is not platform-specific.