diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 87a53966e44046a196fa02d776a2baa43351fbb0..6a181882ae015cc1228da1a9d70a98ac29171604 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -952,7 +952,7 @@ def optimizer_hints(*args)
     def optimizer_hints!(*args) # :nodoc:
       args.flatten!
 
-      self.optimizer_hints_values += args
+      self.optimizer_hints_values |= args
       self
     end
 
diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index 3f370e5ede17a185fe44fa1546c7df5a8965f95a..4b2229e8db2ce3527208efd5e329cd5abe87e373 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -363,6 +363,13 @@ def test_relation_with_optimizer_hints_filters_sql_comment_delimiters
       assert_match %r{/\*\+ BADHINT \*/}, post_with_hint.to_sql
     end
 
+    def test_does_not_duplicate_optimizer_hints_on_merge
+      escaped_table = Post.connection.quote_table_name("posts")
+      expected = "SELECT /*+ OMGHINT */ #{escaped_table}.* FROM #{escaped_table}"
+      query = Post.optimizer_hints("OMGHINT").merge(Post.optimizer_hints("OMGHINT")).to_sql
+      assert_equal expected, query
+    end
+
     class EnsureRoundTripTypeCasting < ActiveRecord::Type::Value
       def type
         :string