Need to raise ImproperlyConfigured exception if basename is not unique

[dgravitate]

When basename is dynamically generated in BaseRouter.get_default_basename, it could (and does) generate a conflicting basename when multiple routes use the same Model. Additionally, developers could inadvertently use a basename that has already been used elsewhere.

In either case, this can lead to unexpected results when (correctly) using reverse() to get the URL

See PR #8438