I have a model that is searchable based on fields that are several levels of indirection away, i.e.
sections = ManyToManyField(Section)
search_fields = ('name', '^sections__instructors__surname',
i.e. Courses refers to Sections which refers to Instructors which has a couple fields to search on. Sections may belong to more than one course, and Instructors may belong to more than one section. In the current trunk, a search on surname in this scheme can return the same Course multiple times.
To fix this, I simply changed
django.contrib.admin.views.main.ChangeList.get_query_set() to return a
QuerySet().distinct() all the time. This doesn't seem to break anything and it fixes the duplicate results problem. However, it's possible it may have some performance impact so you might want to make it conditional on whether or not the search is across joined tables. However, it wasn't really obvious how to do this so I went for the quick-and-(clean|dirty) solution.