posts: add script to remove duplicate votes.

There were 67 duplicate votes in the production database.

script/fixes/075_delete_duplicate_post_votes.rb

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+PostVote.transaction do
+  PostVote.group(:post_id, :user_id).having("count(*) > 1").count.each do |(post_id, user_id), count|
+    votes = PostVote.where(post_id: post_id, user_id: user_id).order(:id)
+
+    # Remove all but the first duplicate vote.
+    dupe_votes = votes.drop(1)
+    dupe_votes.each { p _1 }
+    dupe_votes.each(&:destroy)
+  end
+end