The other day someone was asking if Lucky Egg can stack in Pokemon Go. The answer is yes and no.
Lucky Egg can stack with holiday events that give a bonus to XP. For example, in the 2016 Christmas holiday event, you could get double XP for certain actions. Using a Lucky Egg would double this bonus again, stacking to give a 4x bonus.
However Lucky Egg cannot be stacked on another Lucky Egg.