SortingStyles can swallow items

[Rasmus]

There is no accomodation made e.g. in author_year_title.py for the situation where two entries map to the same key. The sort method from BaseSortingStyle must account for that when doing this:

 entry_dict = dict(
            (self.sorting_key(entry), entry)
            for entry in entries
        )

since values are silently overwritten.