Prevent creation of duplicate poll voters

Note that, when taking votes from an erased user, since poll answers
don't belong to poll voters, we were not migrating them in the
`take_votes_from` method (and we aren't migrating them now either).

spec/controllers/officing/voters_controller_spec.rb

@@ -0,0 +1,27 @@
+require "rails_helper"
+
+describe Officing::VotersController do
+  describe "POST create" do
+    it "does not create two records with two simultaneous requests", :race_condition do
+      officer = create(:poll_officer)
+      poll = create(:poll, officers: [officer])
+      user = create(:user, :level_two)
+
+      sign_in(officer.user)
+
+      2.times.map do
+        Thread.new do
+          begin
+            post :create, params: {
+              voter: { poll_id: poll.id, user_id: user.id },
+              format: :js
+            }
+          rescue ActionDispatch::IllegalStateError
+          end
+        end
+      end.each(&:join)
+
+      expect(Poll::Voter.count).to eq 1
+    end
+  end
+end