[Feature Request] Check if the user already has a book for books_on_join

[AE720]

I'm still looking into an inventory plugin but, if it's feasible at all, it would be nice if the plugin could detect that a user already had a book and if so, not give them a second one. I can imagine it would get tedious having to throw them away. Sometimes they stack but other's I've seen it show up in two spots.