FIX: converting topic to message for a second time was broken

app/models/topic_converter.rb

@@ -49,7 +49,7 @@ class TopicConverter
       @topic.category_id = nil
       @topic.archetype = Archetype.private_message
       add_allowed_users
-      @topic.save
+      @topic.save!
       watch_topic(topic)
     end
     @topic
@@ -77,7 +77,7 @@ class TopicConverter
       user.user_stat.post_count -= 1
       user.user_stat.save!
     end
-    @topic.topic_allowed_users.build(user_id: @user.id)
+    @topic.topic_allowed_users.build(user_id: @user.id) unless @topic.topic_allowed_users.where(user_id: @user.id).exists?
     # update topics count
     @topic.user.user_stat.topic_count -= 1
     @topic.user.user_stat.save!

spec/models/topic_converter_spec.rb

@@ -131,5 +131,17 @@ describe TopicConverter do
         expect(topic.reload.user.user_stat.post_count).to eq(0)
       end
     end
+
+    context 'when user already exists in topic_allowed_users table' do
+      before do
+        topic.topic_allowed_users.create!(user_id: admin.id)
+      end
+
+      it "works" do
+        private_message = topic.convert_to_private_message(admin)
+        expect(private_message).to be_valid
+        expect(topic.archetype).to eq("private_message")
+      end
+    end
   end
 end