Add task to delete duplicate voters

Note that, since poll answers belong to a user and not to a voter, we
aren't doing anything regarding poll answers. This is a separate topic
that might be dealt with in a separate pull request.

Also note that, since there are no records belonging to poll voters, and
poll voters don't use `acts_as_paranoia` and don't have any callbacks on
destroy, it doesn't really matter whether we call `destroy!` or
`delete`. We're using `delete` so there are no unintended side-effects
that might affect voters with the same `user_id` and `poll_id` on
Consul Democracy installations customizing this behavior.

lib/tasks/consul.rake

@@ -7,6 +7,7 @@ namespace :consul do
 
   desc "Runs tasks needed to upgrade from 2.1.1 to 2.2.0"
   task "execute_release_2.2.0_tasks": [
-    "db:mask_ips"
+    "db:mask_ips",
+    "polls:remove_duplicate_voters"
   ]
 end

lib/tasks/polls.rake

@@ -0,0 +1,27 @@
+namespace :polls do
+  desc "Removes duplicate poll voters"
+  task remove_duplicate_voters: :environment do
+    logger = ApplicationLogger.new
+    logger.info "Removing duplicate voters in polls"
+
+    Tenant.run_on_each do
+      duplicate_ids = Poll::Voter.select(:user_id, :poll_id)
+                                 .group(:user_id, :poll_id)
+                                 .having("count(*) > 1")
+                                 .pluck(:user_id, :poll_id)
+
+      duplicate_ids.each do |user_id, poll_id|
+        voters = Poll::Voter.where(user_id: user_id, poll_id: poll_id)
+        voters.excluding(voters.first).each do |voter|
+          voter.delete
+
+          tenant_info = " on tenant #{Tenant.current_schema}" unless Tenant.default?
+          logger.info "Deleted duplicate record with ID #{voter.id} " \
+                      "from the #{Poll::Voter.table_name} table " \
+                      "with user_id #{user_id} " \
+                      "and poll_id #{poll_id}" + tenant_info.to_s
+        end
+      end
+    end
+  end
+end