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/models/poll/answer_spec.rb

@@ -155,6 +155,16 @@ describe Poll::Answer do
 
       expect(Poll::Voter.count).to be 1
     end
+
+    it "does not create two voters when calling the method twice at the same time", :race_condition do
+      answer = create(:poll_answer, question: question, author: author, answer: "Yes")
+
+      2.times.map do
+        Thread.new { answer.save_and_record_voter_participation }
+      end.each(&:join)
+
+      expect(Poll::Voter.count).to be 1
+    end
   end
 
   describe "#destroy_and_remove_voter_participation" do

spec/models/user_spec.rb

@@ -744,6 +744,24 @@ describe User do
       expect(Poll::Voter.where(user: other_user).count).to eq(0)
       expect(Poll::Voter.where(user: user)).to match_array [v1, v2]
     end
+
+    it "does not reassign votes if the user has already voted" do
+      poll = create(:poll)
+      user = create(:user, :level_three)
+      other_user = create(:user, :level_three)
+
+      voter = create(:poll_voter, poll: poll, user: user)
+      other_voter = create(:poll_voter, poll: poll, user: other_user)
+      other_poll_voter = create(:poll_voter, poll: create(:poll), user: other_user)
+
+      expect(Poll::Voter.where(user: user)).to eq [voter]
+      expect(Poll::Voter.where(user: other_user)).to match_array [other_voter, other_poll_voter]
+
+      user.take_votes_from(other_user)
+
+      expect(Poll::Voter.where(user: user)).to match_array [voter, other_poll_voter]
+      expect(Poll::Voter.where(user: other_user)).to eq []
+    end
   end
 
   describe "#take_votes_if_erased_document" do