Fix #3871: Autocomplete can return duplicate tags.

app/logical/tag_autocomplete.rb

@@ -23,7 +23,7 @@ module TagAutocomplete
   end
 
   def count_sort(query, words)
-    words.uniq.sort_by do |x|
+    words.uniq(&:name).sort_by do |x|
       x.post_count * x.weight
     end.reverse.slice(0, LIMIT)
   end

test/models/tag_autocomplete_test.rb

@@ -8,6 +8,11 @@ class TagAutocompleteTest < ActiveSupport::TestCase
       create(:tag, name: "abcdef", post_count: 1)
       assert_equal(["abcdef"], subject.search("A").map(&:name))
     end
+
+    should "not return duplicates" do
+      create(:tag, name: "red_eyes", post_count: 5001)
+      assert_equal(%w[red_eyes], subject.search("re").map(&:name))
+    end
   end
 
   context "#search_exact" do