Add test for answer uniqueness

spec/integration/answer_uniqueness_spec.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe "Answer uniqueness" do
+  let(:user) { FactoryBot.build(:user) }
+  let(:question) { FactoryBot.create(:question) }
+
+  subject { 2.times { user.answer(question, "random") } }
+
+  it "does not allow answering the same question twice" do
+    expect{ subject }.to raise_error(ActiveRecord::RecordInvalid)
+  end
+end