Hi,
I don't know if this circuit has already been invented before (probably has ) but I've never seen it before in any diagram so hmmm... maybe it is new!!
Quite old hat. You'll find it in the CMOS-555 datasheet (and in numerous schematics on the net).
The duty cycle of the output is a constant 50% no matter what the frequency, [...]
If you use a CMOS version of the 555 (7555, 555C etc), you'll get quite close to 50% duty cycle, but with a plain old 555 you won't, as the output is far from rail-rail (and the duty cycle will most likely differ from one chip to another).