Fix #4651: Favorites table contains duplicate favorites

Add fix script to remove duplicate favorites. When a user has duplicate
favorites on the same post, the earliest favorite will be kept and the
rest will be removed.

script/fixes/080_delete_duplicate_favorites.rb

@@ -0,0 +1,22 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+Favorite.transaction do
+  0.upto(99) do |i|
+    Favorite.where("user_id % 100 = #{i}").group(:post_id, :user_id).having("count(*) > 1").count.each do |(post_id, user_id), count|
+      favs = Favorite.where(post_id: post_id, user_id: user_id).order(:id)
+
+      # Remove all duplicates, leaving the oldest favorite.
+      dupe_favs = favs.drop(1)
+      puts "user_id=#{user_id} post_id=#{post_id} count=#{count} keep=#{favs.first.id} drop=#{dupe_favs.map(&:id).join(",")}"
+      dupe_favs.each(&:delete)
+
+      post = Post.find(post_id)
+      post.update_columns(fav_count: post.favorites.count)
+
+      user = User.find(user_id)
+      user.update_columns(favorite_count: user.favorites.count)
+    end
+  end
+end