Would you buy a dual-screen phone? (results)
There are two phones so far in that last category, the Nubia X and the Vivo NEX Dual Display edition. The Vivo is a successor to the one that sort of pioneered the true "all-screen" field by having a pop-up selfie camera onboard, thus negating the requirement for a thicker top bezel to fit one. In addition to its 6.4" OLED screen at the front, however, Vivo slapped a 5.5" second screen on the back of the phone to look yourself up while taking a selfie with the superior rear camera set.
It's only superior to the front cameras of phones like the S9+, Pixel 3 or iPhone XS in some indoor and low-light conditions, though, as our samples attest yet it is a cool idea nonetheless.