This article goes back to the origin of Digital Twin (DT) concepts, contextualizes their emergence and highlights their main key points. Models of DT are discussed with regard to nine criteria: physical asset, model, communication, service, data, product, autonomy, environment, and human. Criteria like autonomy, environment and human are rarely studied in terms of shared control between humans and machines or in terms of user surroundings integration while Human-Centered System Design approaches exist. Few of models studied consider humans as a physical part of DT. Similarly to a Machine Digital Twin, it is proposed to realized a Human Digital Twin for modelling the human part of Human-Machine Systems (HMS). Therefore, design guidelines, solutions and a new framework are proposed to set the pace for a new generation of Digital Twins.