search_fields: duplicated search results when following relationships "backward"

[while0pass]

If in search_fields one uses both fields of a model A and a model B related to A as many to one (B has a foreign key field to A), search results for A-objects may contain multiple occurrence of the same A-object.

Example code:

# models.py
class DictionaryEntry(models.Model):
  ...
  transcription = models.CharField(...)
  ...

class OrthographicVariant(models.Model):
  entry = models.ForeignKey(DictionaryEntry, related_name='orthvars')
  orthvar = models.CharField(...)
  ... 

# admin.py
class AdminDictEntry(admin.ModelAdmin):
  ...
  search_fields = ('transcription', 'orthvars__orthvar')
  ...

[Aymeric Augustin]

This ticket looks very much like #15819, where a discussion has already started.