Raise an error when aggregation alias is conflicting with an annotation

Co-authored-by: Nick Pope <nick.pope@flightdataservices.com>

django/db/models/query.py

@@ -385,6 +385,9 @@ def aggregate(self, *args, **kwargs):
 
         query = self.query.chain()
         for (alias, aggregate_expr) in kwargs.items():
+            if alias in query.annotations:
+                raise ValueError("Aggregation alias '%s' conflicts with an existing annotation." % alias)
+
             query.add_annotation(aggregate_expr, alias, is_summary=True)
             if not query.annotations[alias].contains_aggregate:
                 raise TypeError("%s is not an aggregate expression" % alias)

tests/aggregation/tests.py

@@ -1004,6 +1004,11 @@ def test_annotate_over_annotate(self):
 
         self.assertEqual(author.sum_age, other_author.sum_age)
 
+    def test_same_annotation_aggregate_alias(self):
+        msg = "Aggregation alias 'age_alias' conflicts with an existing annotation."
+        with self.assertRaisesMessage(ValueError, msg):
+            Author.objects.annotate(age_alias=F('age')).aggregate(age_alias=Sum(F('age')))
+
     def test_annotated_aggregate_over_annotated_aggregate(self):
         with self.assertRaisesMessage(FieldError, "Cannot compute Sum('id__max'): 'id__max' is an aggregate"):
             Book.objects.annotate(Max('id')).annotate(Sum('id__max'))