diff --git a/django/db/models/query.py b/django/db/models/query.py
index edb0991e2c..7d41fe1ca6 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -619,8 +619,19 @@ class QuerySet(object):
         with data aggregated from related fields.
         """
         for arg in args:
+            if arg.default_alias in kwargs:
+                raise ValueError("The %s named annotation conflicts with the "
+                                 "default name for another annotation."
+                                 % arg.default_alias)
             kwargs[arg.default_alias] = arg
 
+        names = set([f.name for f in self.model._meta.fields])
+        for aggregate in kwargs:
+            if aggregate in names:
+                raise ValueError("The %s annotation conflicts with a field on "
+                    "the model." % aggregate)
+
+
         obj = self._clone()
 
         obj._setup_aggregate_query(kwargs.keys())
diff --git a/tests/regressiontests/aggregation_regress/tests.py b/tests/regressiontests/aggregation_regress/tests.py
index 51f439c5a1..786431420d 100644
--- a/tests/regressiontests/aggregation_regress/tests.py
+++ b/tests/regressiontests/aggregation_regress/tests.py
@@ -481,6 +481,14 @@ class AggregationTests(TestCase):
             lambda b: b.name
         )
 
+    def test_duplicate_alias(self):
+        # Regression for #11256 - duplicating a default alias raises ValueError.
+        self.assertRaises(ValueError, Book.objects.all().annotate, Avg('authors__age'), authors__age__avg=Avg('authors__age'))
+
+    def test_field_name_conflict(self):
+        # Regression for #11256 - providing an aggregate name that conflicts with a field name on the model raises ValueError
+        self.assertRaises(ValueError, Author.objects.annotate, age=Avg('friends__age'))
+
     def test_pickle(self):
         # Regression for #10197 -- Queries with aggregates can be pickled.
         # First check that pickling is possible at all. No crash = success