Delete duplicate keys in migrate_keys task

lib/tasks/migrate/migrate_keys.rake

 desc "GITLAB | Migrate SSH Keys"
 task migrate_keys: :environment do
   puts "This will add fingerprint to ssh keys in db"
+  puts "If you have duplicate keys https://github.com/gitlabhq/gitlabhq/issues/4453 all but the first will be deleted".yellow
   ask_to_continue
 
   Key.find_each(batch_size: 20) do |key|
     if key.valid? && key.save
       print '.'
+    elsif key.fingerprint.present?
+      puts "\nDeleting #{key.inspect}".yellow
+      key.destroy
     else
       print 'F'
     end
   end
+  print "\n"
 end