Mormonism does not teach that the Son had doubled natures from the beginning. So it’s not them.
@runningdude, Monophysitism teaches that the human nature of Christ combined with his God nature to form one single nature. So it’s not that. And Eutychianism is a form of Monophysitism which teaches that Christ Divinity “absorbed” or “consumed” his humanity. So it’s not that either.
I don’t think there is any heresy that teaches Christ had two natures from the beginning. You may have miss read it or remembered it wrong. That can happen. However if it does exist, it was probably very small and died quickly seeing that not much theologians know about it. Or it was some Gnostic sect that was among the thousands of other Gnostic sects.