Frame doubling is used for both FreeSync and G-Sync at below 40Hz with LCD panels(Not always, but most of the time), it is often the only way to do sub-30(And with most panels sub-40) variable refresh rates, an absolute necessity due to the limitations of LCD technology itself. LCDs need to maintain a minimum true refresh rate to avoid blanking(very very few LCD panels can maintain a true freshrate of lower than 30Hz and this generally has to come at the cost of maximum framerate and/or panel latency.
Frame doubling only exists to force the monitor to display the same image a period of time longer than it can naturally "hold" that image without it fading or blacking out. It is technically what happens whenever you run a monitor at a fixed refresh rate(IE without VRR/FS/GS) but have a variable input rate(Any game with V-Sync disabled) or a set rate below the monitors physical refresh rate(IE a 24 FPS film). There is NO perceivable difference between a monitor pushing 30FPS VRR through frame doubling and a monitor pushing 30FPS VRR natively. The end result is exactly the same. If it doesn't feel smooth, it's because the frame rate is low.
To be clear: LFC only exists to solve one issue, which is the only real issue with VRR/G-Sync/FreeSync below 40/30Hz, which is flickering from the screen blacking out due to the panel attempting to display the same image for too long.