Fix de-duplication of unsaved records for `ActiveRecord::Associations::CollectionProxy#<<`

The existing process was attempting to detect duplicates by storing added records in `Set.new`.
When a record is added to `Set.new`, the identity is calculated using `ActiveRecord::Core#hash`, but this value changes before and after saving, so duplicates were not detected before and after saving even for the same object.

This PR fixed the problem by using the `#object_id` of the record to detect duplicates.
Note that when storing a new object obtained by `ActiveRecord::Base.find` etc., duplicates are not eliminated because the `#object_id` is different. This is the same behavior as the current `ActiveRecord::Associations::CollectionProxy#<<`.

activerecord/lib/active_record/associations/collection_association.rb

@@ -87,7 +87,7 @@ module ActiveRecord
       def reset
         super
         @target = []
-        @replaced_or_added_targets = Set.new
+        @replaced_or_added_targets = Set.new.compare_by_identity
         @association_ids = nil
       end
 

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -802,6 +802,26 @@ class InverseBelongsToTests < ActiveRecord::TestCase
     end
   end
 
+  def test_with_has_many_inversing_does_not_add_unsaved_duplicate_records_when_collection_is_loaded
+    with_has_many_inversing(Interest) do
+      human = Human.create!
+      human.interests.load
+      interest = Interest.new(human: human)
+      human.interests << interest
+      assert_equal 1, human.interests.size
+    end
+  end
+
+  def test_with_has_many_inversing_does_not_add_saved_duplicate_records_when_collection_is_loaded
+    with_has_many_inversing(Interest) do
+      human = Human.create!
+      human.interests.load
+      interest = Interest.create!(human: human)
+      human.interests << interest
+      assert_equal 1, human.interests.size
+    end
+  end
+
   def test_recursive_model_has_many_inversing
     with_has_many_inversing do
       main = Branch.create!