Sadly you can't sell pianos.
But yes, I've had the same happen and have talked to several staff about it.
I have as well, They said they will fix it soon. And hope they do. I feel what ever causes the furniture to duplicate. It causes the ability to steal other people's gralats. Since both can't be done just whenever ( Until someone comes and finds out how to i suppose) . As I seen if you enter their house and happen to enter when the server is lagging, Well i guess you already know what I'm saying. Their gralats appear unlocked to you.