fix duplicate results returned by is_accessible

    For some reason the queries are returning duplicates. It cannot be from the code that checks access through events, because even if the `accessible_projects_ids` has duplicates, the result returned should not have duplicates
    ```
    >>> PublishedProject.objects.filter(id__in=[4,4,4])
        <QuerySet [<PublishedProject: COVID-19 Epidemiology and Vaccination Dataset v1.0.0>]>
    ```

    It looks like it might be because of union operation somehow. I am not sure what exactly, but i have hacky fix with .distinct() (Thanks to @ kshalot)

    On this commit, i added .distinct() to remove the duplicates so that method only returns unique projects/datasets

physionet-django/project/managers/publishedproject.py

@@ -61,4 +61,4 @@ def accessible_by(self, user):
                 accessible_projects_ids.append(event_dataset.dataset.id)
         query |= Q(id__in=accessible_projects_ids)
 
-        return self.filter(query)
+        return self.filter(query).distinct()