I have two adopted children, and I'm sorry to tell you that if our case is any indicator, it isn't an isolated incident. My kids have heard this many times. Sometimes, it isn't meant to be cruel. The child who says it just doesn't know anything about adoption and is trying to figure things out. But sometimes, they are trying to be cruel. Kids can be very, very mean.
Personally, I find the best way to deal with it is to have talked to my children a fair amount about adoption, what it means, what birthmothers are and what mothers are. Then, when they get the mean comments, they will tell the kids right off that they are ignorant and that of course, I'm "real." What else would I be? Imaginary?
So sorry you are dealing with this.