Prevent duplicate filters for encrypted attributes

When an Active Record encrypted attribute is declared, a filter for it
is automatically added to `config.filter_parameters`.  Prior to this
commit, the filter would be re-added every time the model was reloaded:

  ```ruby
  class Post < ActiveRecord::Base
    encrypts :title
  end
  ```

  ```irb
  irb> Rails.application.config.filter_parameters
  # => [:passw, ..., :ssn]

  irb> Post

  irb> Rails.application.config.filter_parameters
  # => [:passw, ..., :ssn, "post.title"]

  irb> reload!
  irb> Post

  irb> Rails.application.config.filter_parameters
  # => [:passw, ..., :ssn, "post.title", "post.title"]
  ```

This commit ensures filters are only added once so that
`config.filter_parameters` does not grow unbounded.

activerecord/lib/active_record/encryption/configurable.rb

@@ -48,11 +48,11 @@ def encrypted_attribute_was_declared(klass, name) # :nodoc:
           end
         end
 
-        def install_auto_filtered_parameters_hook(application) # :nodoc:
+        def install_auto_filtered_parameters_hook(app) # :nodoc:
           ActiveRecord::Encryption.on_encrypted_attribute_declared do |klass, encrypted_attribute_name|
-            filter_parameter = [("#{klass.model_name.element}" if klass.name), encrypted_attribute_name.to_s].compact.join(".")
+            filter = [("#{klass.model_name.element}" if klass.name), encrypted_attribute_name.to_s].compact.join(".")
-            unless excluded_from_filter_parameters?(filter_parameter)
+            unless excluded_from_filter_parameters?(filter)
-              application.config.filter_parameters << filter_parameter
+              app.config.filter_parameters << filter unless app.config.filter_parameters.include?(filter)
               klass.filter_attributes += [encrypted_attribute_name]
             end
           end

railties/test/application/initializers/frameworks_test.rb

@@ -376,6 +376,27 @@ def self.<(_)
       assert_nil ActiveRecord::Scoping::ScopeRegistry.current_scope(Post)
     end
 
+    test "filters for Active Record encrypted attributes are added to config.filter_parameters only once" do
+      rails %w(generate model post title:string)
+      rails %w(db:migrate)
+
+      app_file "app/models/post.rb", <<~RUBY
+        class Post < ActiveRecord::Base
+          encrypts :title
+        end
+      RUBY
+
+      require "#{app_path}/config/environment"
+
+      assert Post
+      filter_parameters = Rails.application.config.filter_parameters.dup
+
+      reload
+
+      assert Post
+      assert_equal filter_parameters, Rails.application.config.filter_parameters
+    end
+
     test "ActiveRecord::MessagePack extensions are installed when using ActiveSupport::MessagePack::CacheSerializer" do
       rails %w(generate model post title:string)
       rails %w(db:migrate)