I think you have to tell her to cancel the order, whether it's unpleasant or not. Seriously, what are you going to do with two playhouses? It's not like an extra truck or something that you could give to someone else. And returning it will likely be quite difficult once you have it (although if it's too late to cancel you can refuse delivery and have it sent back to the shipper at no charge to you).
Obviously it's all about HOW you go about it, so I'd start with Thank you, that's such a generous and thoughtful gift, but I'm sorry, we already have a playhouse. And then reiterate some of your other suggestions for outside toys.
My inlaws are often times clueless and do stuff like this. Get things that are kind-of like what my son wants, or just completely random inappropriate stuff -- after asking for suggestions. Who knows what's going on in their heads, but hopefully their intentions are to do something nice for your son and they'll be able to find another way to do that. Good luck.