FIX: Ensure unique fields in TopicList.preloaded_custom_fields.

app/models/topic_list.rb

@@ -4,7 +4,7 @@ class TopicList
   include ActiveModel::Serialization
 
   cattr_accessor :preloaded_custom_fields
-  self.preloaded_custom_fields = []
+  self.preloaded_custom_fields = Set.new
 
   attr_accessor :more_topics_url,
                 :prev_topics_url,
@@ -20,6 +20,8 @@ class TopicList
     @current_user = current_user
     @topics_input = topics
     @opts = opts || {}
+
+    preloaded_custom_fields << DiscourseTagging::TAGS_FIELD_NAME if SiteSetting.tagging_enabled
   end
 
   def preload_key
@@ -81,11 +83,8 @@ class TopicList
       ft.topic_list = self
     end
 
-    preload_custom_fields = TopicList.preloaded_custom_fields
+    if preloaded_custom_fields.present?
-    preload_custom_fields << DiscourseTagging::TAGS_FIELD_NAME if SiteSetting.tagging_enabled
+      Topic.preload_custom_fields(@topics, preloaded_custom_fields)
-
-    if preload_custom_fields.present?
-      Topic.preload_custom_fields(@topics, preload_custom_fields)
     end
 
     @topics

spec/models/topic_list_spec.rb

@@ -0,0 +1,35 @@
+require 'rails_helper'
+
+describe TopicList do
+  let!(:topic) { Fabricate(:topic) }
+  let(:user) { topic.user }
+  let(:topic_list) { TopicList.new("liked", user, [topic]) }
+
+  after do
+    TopicList.preloaded_custom_fields.clear
+  end
+
+  describe ".preloaded_custom_fields" do
+    it "should return a unique set of values" do
+      TopicList.preloaded_custom_fields << "test"
+      TopicList.preloaded_custom_fields << "test"
+      TopicList.preloaded_custom_fields << "apple"
+
+      expect(TopicList.preloaded_custom_fields).to eq(Set.new(%w{test apple}))
+    end
+  end
+
+  context "DiscourseTagging enabled" do
+    before do
+      SiteSetting.tagging_enabled = true
+    end
+
+    after do
+      SiteSetting.tagging_enabled = false
+    end
+
+    it "should add tags to preloaded custom fields" do
+      expect(topic_list.preloaded_custom_fields).to eq(Set.new([DiscourseTagging::TAGS_FIELD_NAME]))
+    end
+  end
+end