diff --git a/django/contrib/contenttypes/generic.py b/django/contrib/contenttypes/generic.py
index 23abe01f51..c5137877cb 100644
--- a/django/contrib/contenttypes/generic.py
+++ b/django/contrib/contenttypes/generic.py
@@ -65,7 +65,7 @@ class GenericForeignKey(object):
     def get_prefetch_query_set(self, instances):
         # For efficiency, group the instances by content type and then do one
         # query per model
-        fk_dict = defaultdict(list)
+        fk_dict = defaultdict(set)
         # We need one instance for each group in order to get the right db:
         instance_dict = {}
         ct_attname = self.model._meta.get_field(self.ct_field).get_attname()
@@ -75,7 +75,7 @@ class GenericForeignKey(object):
             if ct_id is not None:
                 fk_val = getattr(instance, self.fk_field)
                 if fk_val is not None:
-                    fk_dict[ct_id].append(fk_val)
+                    fk_dict[ct_id].add(fk_val)
                     instance_dict[ct_id] = instance
 
         ret_val = []
@@ -325,7 +325,7 @@ def create_generic_related_manager(superclass):
             query = {
                 '%s__pk' % self.content_type_field_name: self.content_type.id,
                 '%s__in' % self.object_id_field_name:
-                    [obj._get_pk_val() for obj in instances]
+                    set(obj._get_pk_val() for obj in instances)
                 }
             qs = super(GenericRelatedObjectManager, self).get_query_set().using(db).filter(**query)
             return (qs,
diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index c82c397288..8c054e7303 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -237,7 +237,7 @@ class SingleRelatedObjectDescriptor(object):
         return self.related.model._base_manager.using(db)
 
     def get_prefetch_query_set(self, instances):
-        vals = [instance._get_pk_val() for instance in instances]
+        vals = set(instance._get_pk_val() for instance in instances)
         params = {'%s__pk__in' % self.related.field.name: vals}
         return (self.get_query_set(),
                 attrgetter(self.related.field.attname),
@@ -316,7 +316,7 @@ class ReverseSingleRelatedObjectDescriptor(object):
             return QuerySet(self.field.rel.to).using(db)
 
     def get_prefetch_query_set(self, instances):
-        vals = [getattr(instance, self.field.attname) for instance in instances]
+        vals = set(getattr(instance, self.field.attname) for instance in instances)
         other_field = self.field.rel.get_related_field()
         if other_field.rel:
             params = {'%s__pk__in' % self.field.rel.field_name: vals}
@@ -463,7 +463,7 @@ class ForeignRelatedObjectsDescriptor(object):
             def get_prefetch_query_set(self, instances):
                 db = self._db or router.db_for_read(self.model)
                 query = {'%s__%s__in' % (rel_field.name, attname):
-                             [getattr(obj, attname) for obj in instances]}
+                             set(getattr(obj, attname) for obj in instances)}
                 qs = super(RelatedManager, self).get_query_set().using(db).filter(**query)
                 return (qs,
                         attrgetter(rel_field.get_attname()),
@@ -546,7 +546,7 @@ def create_many_related_manager(superclass, rel):
             from django.db import connections
             db = self._db or router.db_for_read(self.model)
             query = {'%s__pk__in' % self.query_field_name:
-                         [obj._get_pk_val() for obj in instances]}
+                         set(obj._get_pk_val() for obj in instances)}
             qs = super(ManyRelatedManager, self).get_query_set().using(db)._next_is_sticky().filter(**query)
 
             # M2M: need to annotate the query in order to get the primary model