Added documentation about different results according to order of annotate() and filter() in a query.

docs/topics/db/aggregation.txt

@@ -425,6 +425,27 @@ publisher's that have at least one book with a rating exceeding 3.0. The second
 query asks for the average of a publisher's book's ratings for only those
 ratings exceeding 3.0.
 
+Using multivalued filtering against a previously annotated aggregation might
+still yield duplicate results.
+
+Given:
+
+* Publisher A has two books with each having 100 pages.
+
+Here's an example demonstrating this behavior::
+
+    >>> a = Publisher.objects.annotate(total_pages(Sum("books__pages"))
+    >>> a.total_pages
+    200
+    >>> b = Publisher.objects.annotate(Sum("books__pages")).filter(books__in=Books.objects.all())
+    >>> b.total_pages
+    400
+
+This is behavior is caused because ``filter()`` calls won't reuse multivalued
+JOINS generated by previous ``annotate()`` or ``filter`` calls. To prevent issues,
+you should filter after annotations (not the other way around) or use
+``Aggregate.filter``.
+
 It's difficult to intuit how the ORM will translate complex querysets into SQL
 queries so when in doubt, inspect the SQL with ``str(queryset.query)`` and
 write plenty of tests.