Edit: This is the trouble with multiquote. It's got a mind of its own. It doesn't behave properly and I end up faffing around for far too long trying to get it to do what I want. If it did behave I'd probably comment on more photos than I do because they're all fabulous.
Here's the trick. MQ behaves properly if you use it by itself, not combined with other buttons in a post. If you use MQ and QUOTE, you'll get duplicate posts in your typing window. Just do this: MQ . . . MQ . . . MQ . . . POST TO THREAD.