Don't want to duplicate untouched values on merge

I'm ok to allow explicit `select(:id, :id)` duplication, but it is not
the expected behavior that values that users has not touched are
duplicated each time `merge` is called.

Author: kamipo

activerecord/lib/active_record/relation/merger.rb

@@ -85,7 +85,7 @@ def merge_select_values
           return if other.select_values.empty?
 
           if other.model == relation.model
-            relation.select_values += other.select_values
+            relation.select_values += other.select_values if relation.select_values != other.select_values
           else
             relation.select_values += other.instance_eval do
               arel_columns(select_values)

activerecord/test/cases/relation_test.rb

@@ -47,10 +47,11 @@ def test_multi_values_deduplication_with_merge
         unscope:   [ :where ],
         extending: [ Module.new ],
         with:      [ foo: Post.all ],
+        select:    [ :id, :id ],
       }
       expected.default = [ Object.new ]
 
-      (Relation::MULTI_VALUE_METHODS - [:select]).each do |method|
+      Relation::MULTI_VALUE_METHODS.each do |method|
         getter, setter = "#{method}_values", "#{method}_values="
         values = expected[method]
         relation = Relation.new(FakeKlass)
@@ -293,15 +294,6 @@ def test_relation_merging_with_joins_as_join_dependency_pick_proper_parent
       assert_equal 3, relation.where(id: post.id).pluck(:id).size
     end
 
-    def test_merge_preserves_duplicate_columns
-      quoted_posts_id = Regexp.escape(quote_table_name("posts.id"))
-      quoted_posts = Regexp.escape(quote_table_name("posts"))
-      posts = Post.select(:id)
-      assert_queries_match(/SELECT #{quoted_posts_id}, #{quoted_posts_id} FROM #{quoted_posts}/i) do
-        posts.merge(posts).to_a
-      end
-    end
-
     def test_merge_raises_with_invalid_argument
       assert_raises ArgumentError do
         relation = Relation.new(FakeKlass)