Merge pull request rails#37457 from sinsoku/fix_issue_37446

Fix an issue with duplicate preloaded records

Author: kamipo

activerecord/lib/active_record/associations/preloader.rb

@@ -112,7 +112,7 @@ def preloaders_for_hash(association, records, scope, polymorphic_parent)
           association.flat_map { |parent, child|
             grouped_records(parent, records, polymorphic_parent).flat_map do |reflection, reflection_records|
               loaders = preloaders_for_reflection(reflection, reflection_records, scope)
-              recs = loaders.flat_map(&:preloaded_records)
+              recs = loaders.flat_map(&:preloaded_records).uniq
               child_polymorphic_parent = reflection && reflection.options[:polymorphic]
               loaders.concat Array.wrap(child).flat_map { |assoc|
                 preloaders_on assoc, recs, scope, child_polymorphic_parent

activerecord/test/cases/associations/cascaded_eager_loading_test.rb

@@ -190,4 +190,14 @@ def test_eager_association_loading_with_cascaded_interdependent_one_level_and_tw
     assert_equal 3, authors[1].posts.size
     assert_equal 3, authors[0].posts.collect { |post| post.categorizations.size }.inject(0) { |sum, i| sum + i }
   end
+
+  def test_preloaded_records_are_not_duplicated
+    author = Author.first
+    expected = Post.where(author: author)
+      .includes(author: :first_posts).map { |post| post.author.first_posts.size }
+    actual = author.posts
+      .includes(author: :first_posts).map { |post| post.author.first_posts.size }
+
+    assert_equal expected, actual
+  end
 end