In the case of a circular symlink, technically it's still correct to return
non-nil.

```
ls -l /tmp/foo

total 0
lrwxrwxrwx 1 me me 9 Sep  1 10:59 foo -> /tmp/foo/
```

In this case:
```
;;; because
(file-equal-p "/tmp/foo/foo/" "/tmp/foo/")
=> t

;;; therefore

(my-parent-directory "/tmp/foo/foo/") ; (the equal version)
=> "/tmp/foo/"

(file-parent-directory "/tmp/foo/foo/")
=> nil

```

Obviously, the `equal` version fits our expectation better, although that's a
mistake at the time of link creation, "/tmp/foo/foo"'s parent still exists and
not a nil value.

Not to mention `file-equal-p` needs an active TRAMP connection for remote files.

-- 
Daanturo.