This is not uncommon - and she knows this doll is a doll. Apparently, this "pretense" is a sign of a creative and bright child (or so I've read). In a few months, perhaps when school returns - she will forget about this "pretend" friend. I once referred to our granddaughter's friend as "imaginary" and she replied "no, she's real, just invisible". She knew and so did I. All the best ...
yup...normal ...has the other twin any doll friends ?
Are you the child's primary caregiver? If not, when you say "what do we do?" do you mean how should you react? Join in! She knows it is just a doll. She is just getting a kick out of playing with a life-sized avatar.