Prevent duplicate records when preloading has_many

When preloading a has_many association, we were simply concatenating
the preloaded records without regard to whether they were already
loaded on the owner. Even though there is a check for `loaded?` in this
part of the preloader, some persisted records may not be marked as such.
For example, if a record is created via `owner.association.create`. This
change reverts to the previous behavior of replacing the target
association while also preserving non-persisted records, which was the
goal of #50129.

Co-authored-by: John Hawthorn <john@hawthorn.email>

activerecord/lib/active_record/associations/preloader/association.rb

@@ -247,8 +247,8 @@ def associate_records_to_owner(owner, records)
             association = owner.association(reflection.name)
 
             if reflection.collection?
-              association.loaded!
-              association.target.concat(records)
+              not_persisted_records = association.target.reject(&:persisted?)
+              association.target = records + not_persisted_records
             else
               association.target = records.first
             end

activerecord/test/cases/associations_test.rb

@@ -804,6 +804,17 @@ def test_preload_makes_correct_number_of_queries_on_relation
     end
   end
 
+  def test_preload_does_not_concatenate_duplicate_records
+    post = posts(:welcome)
+    post.reload
+    post.comments.create!(body: "A new comment")
+
+    ActiveRecord::Associations::Preloader.new(records: [post], associations: :comments).call
+
+    assert_equal post.comments.length, post.comments.count
+    assert_equal post.comments.all.to_a, post.comments
+  end
+
   def test_preload_for_hmt_with_conditions
     post = posts(:welcome)
     _normal_category = post.categories.create!(name: "Normal")