I can understand having two 'book stacks' to decorate one's house with.

I don't see the point of having two Diarama items, but whatever. I guess somebody out there can find an obscure use for having two of them.

But what about the giant pencil (sword) and the battery (cape)? We can't even HAVE two of them?

Not much can be done about the battery cape, but what about making it so that if you own the second pencil it gives the first pencil an ability, like a moderate reduction on nothings like the Poseidon Trident or something like that?