I have RS Flipflop type of logic(both in hardware & software) successfully using quadra encoder.
It is lkie this
Say encoder prod 2 phase PH1 & PH2

if PH1 stat changes 0 to 1
then count = count +1
wait for PH1 stat to change from 0 to 1
loop
unless and until phase 2 changes stat multiple pulses on phase 1 (due to effect of threshold & vibration) will not have any effect on counter.

Here logic is that one phase transition can increment counter once only and it has to wait for transition of second phase to enable counting again