Was using RX570 on an intel 4480 last week with windows 7 and I had the desktop monitor and TV cloned. I could launch any full screen game on my monitor (1920x1080) and it would be displayed perfectly on my TV (1920x1080) through HDMI cable.
Upgraded to an Asus mobo with Ryzen 7 2700x, kept the RX570, installed Windows 10, launched a game, and it displays fine on monitor, but all it does on TV is flash black for a second when the game is loading and then displays the desktop. I've tried fiddling with all the settings with no success.
I should also add that if I "extend" the monitors so they act as one desktop, if I make the TV the primary display and launch a full screen game it works fine.
I just bought a new monitor and found what was causing the problem with the old monitor. The old monitor was connected to the RX570's DVI port. New monitor has an HDMI connector. RX570 only has one HDMI output so I bought a display port to HDMI converter cable. My TV is now connected to the RX570 HDMI port, and the new monitor is connected via HDMI/display port cable to one of the RX570 display port connectors. I can now duplicate monitors and when I start a game it shows on both screens.
There must be something different about how the hardware/software handles the DVI port as opposed to the display ports/HDMI port.