Apple is reportedly already working on the iPhone Duo 2, which may add an under-display Face ID sensor on the outer screen: https://www.pcmag.com/news/iphone-duo-2-prototypes-reportedly-trial-under-display-face-id
PCMAGiPhone Duo 2 Prototypes Reportedly Trial Under-Display Face IDFuture iPhone Duo models may also receive a larger inner screen and a telephoto camera, says Bloomberg's Mark Gurman.